Since you have the total GPM for the zone you can get very close in to the precip rate by using this formula:
(96.25 x total GPM) \ Total zone area = precip rate
You could also use this website:
It will calculate everything for you as long as you have the catch cup size in sq in.
Me I just looked at my water meter, ran it for 10 min and calculated precip rate that way, faster than catch cups and probably a little more accurate. I wasn't to concerned with effic. since I can tweek that and the depletion % to get the results I want.
I do agree with you that this entire process should be much easier to use or at the least much easier to find instructions on how to use it. My hope is that in the next release that they make this entire thing a behind the scenes calculation and you just put in numbers and it does it for you. That way for all the newbie's it will be easier.