Take up golf. Although this would be a very expensive, long term, time consuming and frustrating solution. If your simply looking to do a better job of guessing a given distance it would probably help though.
Loft on clubs are such that there is roughly 10 yards difference on each club. So, for me, 9 iron = 120y, 8 iron = 130y, 7 = 140y, etc.
If you play enough golf you eventually get fairly good at eyeballing distance. I sued to us a scope range finder for golf and now feel I do not need it as my guessing seem to be locked in.
Although I am used to gauging distance looking at a defined height flag, not a smaller round target, so I have yet to see how well this guessing will translate to archery (I am new to archery).
Although given the lethalness of archery and the frustration level of golf, perhaps it is not such a good idea to mix the two.:wink:
Other than that I do not know any good eyeballing methods