I have wondered this also MMMmmm... My formula is up to .60 sized, I used standard-size 40 +_ servos. After that whatever the MFG recommends, I can afford, and a simple length X width = total area method. On my P-Wag Extra 300, I could've gotten away w/ 70 or 80's, but I used 100 oz's as a measure of safety. Now if you are into all this new exotic flying going on nowadays, you might consider .60-sized ones in a .40-sized plane and so on. Step it up one notch so to speak..... Rob


P.S., I think we all have seen or experienced situations when the servo wasn't up to par___ won't happen again I bet...