When using engine as distributor test machine, it is helpfull to reset timing to 0 degrees and use lowest possible idle speed. Turn on heater fan and head lights,this uses Alternator to place load on engine,helps steady idle speed.
Your measure ments are close enough.Not much advance past 20 degrees,yes thats about right. Short answer use distributer as is except change vaccum advance to the 8.5 one 17 degrees instead of 22.
The only reason I used the 11 vacuum advance can is that my 8.5 can leaks vacuum, about an inch per second. I guess it might work OK but it would be hard to test with a handheld vacuum pump, and I wouldn't have confidence running it. It seems all the aftermarket cans are 8.5. I guess I'll get one of those.