The option that we are considering is to spray foam the ceiling in the attic with blown-in additional insulation. So we'll have ventilation in our attic. And I have several questions:

I know that code says something like 1/300 or 1/150 sq ft of ventilation per sq ft of ceiling if a vapor barrier is used (I think).

I have attached a picture of our roof lines and the locations of the ridge vents.
The blue line represents the exterior perimeter of the house (1924 sq ft), the pink line is the perimeter of the garage (755 sq ft). The green lines represents where we have continuous vented soffit on the house, and the orange is where the vented soffit is on the garage.
These are approx. sizes from the drawings, I haven't really measured the actual length of the ridge vents:
1. 6ft
2. 10 ft
3. 12 ft
4. 12 ft
5. 6 ft
6. 8 ft

So, for an example using the 1/150 req. for the house attic 1924/150 = 12.83. Does that mean that I need at least 12.83 sq foot at the soffit and 12.83 sq ft of total ventilation at the ridges? Or do you split that number in half and I need at least 6.4 sq ft at soffits and 6.4 sq ft at ridges?

Since the garage attic and main house attic are connected, I should be adding the garage sq ft to the calculation, though we are discussing the option of building the wall between the garage and house as a fire rated wall, and go all the way to the roof, so not sure at this point what we are doing.

And how do you know how much ventilation soffit vents and ridge vents do actually provide? Do you go with the vent number for the soffit, or do you count intake ventilation as the ventilation numbers that the baffles between the rafters provide? Which brings me to another question, do you put baffles in every cavity between trusses wherever there are soffit vents, or do you put one every so many rafter bays depending on the amount of ventilation you need?
How balanced do you want your soffit to ridge ventilation? Can you have more intake ventilation than exhaust ventilation?

Due to the complexity of our roof lines, I'm concerned that soffit vents and ridge vents would not be adequate to provide good ventilation in the attic.
Ridge vent #3 is in a vaulted livingroom, and I'm wondering if this ridge vent will really do anything, since the "soffits" of this vaulted roof are all inside the attic.
Also, on the front of the house, there is a covered porch. Should I add some "soffit" vents in the covered porch ceiling along the exterior wall (shown in yellow in the drawing) to sort of balance things out?

Am I splitting hairs and worrying about things too much? Any sort of guidance will be greatly appreciated. Thanks.

In your 12.83 sq/ft calculation that is the total, for a balanced system you would split that number in half so 6.415 sq ft intake and 6.415 sq ft exhaust. Now you multiply that by 144 to get your net free area in inches which is how products are labeled, in your case a balanced roof of 923.76 sq in intake and 923.76 exhaust.
As an example the stuff I use at the ridge gives me 18 net free per foot so using your ridges except for #7 which is in the garage and will most likely get walled off you could get 46' @ 18" per foot so 828 sq in net free at the ridge which I would call good but others may beg to differ.

Problem with hip roofs is that you'll have way more intake venting than you need for a balanced system using all vented soffits which I'm guessing you did. Looking at your drawing you may roughly have 140' of 2' soffit not counting garage which would be 280 sq feet at maybe 5" per square foot if its vented aluminum so your intake is 1400 sq inches and your exhaust is 828 sq inches, if you wanted to balance that you would need a powered fan or 11-12 standard pot vents.
Making it worse is your ridges 5 and 3 probably aren't going to be real effective but with your ceiling vapor barrier you would be allowed the 1/300 reducing your requirement.

I build and remodel and try to balance venting as part of a quality I try to offer but I can bet you most homes built arent balanced at all. A big part of the reason is vented aluminum soffit: skipping vented pans with solid pans looks funny, hip roofs generally dont offer enough ridge, pot vents look horrible so people dont put them in and venting hips halfway down also looks horrible. If you really wanted close to a balanced system then I would switch out vented soffit pans for solid pans to cut down your intake evenly around the house.

I really cant say how well the balanced system works, hopefully someone with more experience has taken temp readings in a balanced vs. unbalanced and chimes in.

As far as the vent chutes - do every space. I dont know how cold your area gets but here we dont want any insulation touching the roof deck as heat will transfer through and start ice dam issues. Think of isolating the roof deck and insulation as a priority.

Thanks greentree.
Uhm, I guess not knowing any different,we took continous soffit vent to mean litterally continous soffit vents, so we don't have any solid soffit at all (except on the gable soffits). And we did it ourselves. It's all vented. So I must deffinitely have more intake ventilation. Do they make powered fans you can attach inside the attic to exhaust through existing ridge vents. Wouldn't that help? It wouldn't require anything ugly outside.
If I definitely have more intake, and say the ridge amount of ventilation would be good enough, could I block the soffit vents to reduce intake?
Since we have not set the decision on stone to do the spray foam in the ceiling, I was thinking that worst case scenario is I need to meet the minimum 1/150. Because if we end up with the vapor retarder on that ceiling, I figured more ventilation can't be worse than less ventilation.

I was thinking that if I had to put vent baffles based on the amount of ventilation that I actually needed, and say I ended up needing vent baffles every other or every third space, I was planning on blocking the ones without baffles and using spray foam really thick so anything blown in wouldn't touch the deck. And I'm going to do such a great job up there that no heat or air will escape my house into the attic (or at least die trying!) *wink*

Thanks for the insight.

Also, I came across a reference that said that the Home Ventilation Institute recommends 60% of the ventilation be intake, and 40% be exhaust instead of a 50/50. Any comments on that?

Thanks.