I think the % coverage info is actually target more at year round users (like in FL and CA) where the idea is to be able to swim year round even when average temps are in the low to mid-70s. Also, the worse the orientation, the more coverage you need.

In the NE, coverage can be less because we only turn the thing on during late spring/summer/early fall when the temps are higher. HOWEVER, as I have stated elsewhere, the nicest thing about more coverage is better recovery time. My 20x40 IG could easily get to the low 90s - when the stupid sun shines! Right now, due to all the rain and sparse sun, we are in the high 70s! (Still better than our neighbors, but not good enough). I would love to have twice the coverage just so that I could get back into the 80s in a day or two instead of 3 or 4. That is probably next years project (I just have to decide if I am going to add 5 panels to the back side of the garage, or GO FOR IT and add another 8 panels to the front roof).