What did you end up doing, OP?
I agree that distances between 3' and maybe 20' are difficult.
What I do is remember that my foot is exactly a foot long. That works for very close distances when I have the time to pace it off. Then I imagine a 6' person laying down and look at the ground.
Another thing that helps is to shoot 400 speed film, so you can use the smaller apertures and it's not so critical. I'm finding 400 film is just a perfect compromise. I can still shoot at dusk and hand-hold, and in bright light, f/16 and 1/500 is usually enough. (and if it's close, rely on film latitude
I have a rangefinder I use for developing my hold-over charts for my airgunning. I think it was about $100, but very accurate. Probably carrying a 25' tape measure a few times until you get the hang of things would be more practical.