Yes, the correct answer is 2700/2.89 as you indicated and is NOT 2700/3.619 as John Siegenthaler indicated. There should be a "+" before the 0.0103 in equation 2.7. We actually pointed out that error to John Siegenthaler several years ago...along with several other errors too. Please also be aware that you should use the average slab temp if the slab is heated and NOT the indoor room temp. Please note that we use the correct Siegenthaler equation in the heat loss software on our website:
Borst Heat Loss Analysis Software
I should also note that the hydronic radiant floor heating design software on our website assumes that the exposed floor heat loss is calculated using the indoor room temp as would normally be done, and the hydronic radiant floor heating design software then subsequently factors this exposed floor heat loss to be higher based on the design average slab temp as determined by the software. This make more sense since you don't know what this average slab temp will be until you actually do the design... We found this to be a deficiency in John Siegenthaler's hydronic radiant design software several years ago as well and we refined our hydronic radiant floor heating design software to better address this:
Borst Hydronic Radiant Floor Heating Design Software
Anyhow, I hope this helps and you are no longer at a stand still, LOL!