Uh, I agreed with you. We're just going at it differently. Your 5 off at 60 is the same as my 8-10% UN spec. I gave the OP a range initially. The 5% low error you also posted.

"Magical Math" is only a starting point, I agree you have to get out and actually test.

Bottom line, I'm agreeing with you, we're just looking at it differently.

EDIT: Just saw your second post, I just think we're saying the same basic things in different ways. It's all good.