That would make no sense. It wouldn't negatively affect a tank to have shell hits 70-90 degrees being truncated at 70, infact it would help the tank, since hits impacting flat on the armor would register as hits at 30degrees off of perpendicular. Somethin like this: \\ <== as opposed to [] <== .
See what I'm saying? I think AWwrgwy is correct. Shells impacting perpendicular to the armor are 0 degrees, while shell hits parralel to the armor are 90 degrees