And even that is a waste of time (in more ways than one, but I digress). If I'm five radials off and the correction factor is 3.5, I'd say the answer is somewhere between 15 and 20 and call it a day. As long as a stud could tell me how to do it and then flew something close, I was happy. It's not like you can fly 17.5 degrees the whole time, anyway.
I suppose it varies directly with what kind of IP you fly with...
"Ok, sir. I'm estimating 15 degrees"
"Oh, REALLY, EN-sign?? I think it's more like 17!"