There are a couple of things that you can do to get the "theoretical" pin gaps
1) Use a program like On-Target, Archers Advantage or The Archery Program (TAP)
Based on the information you put in, you can get some very accurate sight tapes out based solely on dead reconing and physics. The better the information in, the more accurate the sight tape out. If you can get a pin set at 20 and one at 60 or longer you can generate an extremely accurate tape that should cover all distances in between.
2) Use a preprinted sight scale. Same principle as above, get a 20 and 60 yard pin sighted dead on, then compare your pin settings to the various scales on the tape. When you find one where the pins line up exactally, all of the intermediate yardages should be correct.
In my experience, using these methods is extremely accurate in getting your intermediate yardages set. I have found that as long as my bow is tuned properly, the 20/60 gap method works very well. If you find that given a good 20 and good 60 pin your intermediate yardage pins aren't on the money, then you need to look into something like tuning or fletching contact.
All in all, this is really just a roughing in process. As the others have stated, you still have to shoot every distance to ensure that things are dialed in.