John,I think this is a very useful question and the answers would be very helpful to many archers.
If you have the right arrows, no worries, but what if you have the wrong ones?
At 20y the bare shaft is X away from the fletched ones. Does X require one spine difference to sort out or two; or three???
My guess is that the worse the match, the bigger the distance between fletched and bare shaft.
I am going to stick my neck out and say that 3 to 4 inches would be a one spine difference. Bigger than that is more than one spine. I know that sounds basic and not really helpful but, what happens in real life archery?
I think most archers buy their arrows; then test them and have great results or a close match or rubbish match.
Close match will be good enough for many. One change of spine will get the close matches to be great matches for those who look for better.
Those with rubbish results will probably look at the charts again and find they have misread something.Or they may get advice and try a two spine change because their results look worse than the one spine difference.
Put another way; if your results later prove to be five spines wrong, I feel the," first time results", could be very difficult to read anyway.