well, actually, I have tried to devlop objective criteria--
after a fashion. This "light" and "heavy" description bothered me when I first started revolver loading two years ago. It particularly bothered me afer I realized I had no bullet jump in that 640 / 38-Special-17+grains-of-AA#7-under-a-140LTC experience. Yes, I had been applying a "heavy" crimp.
So, I did set out to try to quantify it, besides the "look at it" routine. What I have found is that, given my particular and fine-tuned swing, four raps or more to remove the bullet with an inertial hammer means it is a heavy crimp.
Personally, I use shorter 'raps,' not full-arm swings when I use the hammer. And, I use a wooden block on the concrete floor under it.
Keep in mind that the crimp amount will vary by the bullet's characteristics. For example, the Speer 158LSWC-HPs appear to be 'undersized' and one has to noticably step up the crimp, as compared to the house-brand 158LSWCs from my LGS. IOW, you need to set that crimp for each bullet you use--and it may vary not only by brand but also by bullet-production runs.
Given the fact that I use a Lee FCD as my 4th / crimping die, I have been able to tweak its settings--particularly the body height--so that a full range of crimp can be dialed in, once it has been set for the bullet being used.
Two years later, I do mostly rely on a visual inspection--but each time I change out bullets, etc., and go through tweaking the crimp, I do double-check with the "four-raps-plus" test.
Jim H.