Prior to a working densitometer being gifted to me (a MacBeth TD504, that so far seems to work just fine), I would (for larger negative sizes) place the neg and a 6x6cm size step wedge on a light table. I would read the neg area on my pentax 1 degree spot meter from the camera bag, and then scan the step wedge unti I came up with the right step that matched the expsoure reading. I made a cardboard mask after I realized the meters' lens flare would throw meter readings off when I was taking readings near the edge of the neg or step wedge.

It was feasible for 120, easy on 4x5, but hopeless on 35mm.