That is actually the opposite of what I remember about the rules, that using your body to measure the distance is OK, just no equipment. So putting in say 3 fingers in the gap, or using your fist to measure the distance is OK. Because it is not a precise measurement, at least that is my view of it. I don't know how others feel, but I think there should be some difficulty to this game, and using equipment to measure gaps to me feels like it makes things a bit easier than the skill reequipment to play should be. Measuring using things like a fist, fingers, something organic and not precise, is a bit more natural.
My view on what should be OK and what is not, is generally based on the idea that if some tool/equipment/device allows you to do something very hard or about impossible and makes it very easy, it should not be in the game. So using a ball to measure the space, very easy, it's known size and is exact, makes the measurement very easy. Fist, not so much, it's a fairly rough estimate. Almost the exact thing I have against jump cues, they make the normally very hard act of getting over a ball trivial due to technology, and in some cases make some absolutely impossible shots possible (say jumping form a ball width or two away), thus cheapening the effort and skill of the game. I have as little interest in executing a shot or winning due to a gimmick as I do from winning by cheating or hiding a foul.