Is their an ideal flow rate or should I just measure EWT and LWT . of 250’ half inch pex loops?

what are you trying to do?

typically, a temperature drop across the loop of 10 to 30 degrees is used, with 20 being an arbitrary residential max. however, supply temperature restrictions/requirements, balancing between zones might change the flow rates you are looking to achieve.

Thermally, more flow is better and I wouldn't cause any flow restriction that wasn't required for balancing.