How to measure 1/4" per 1' slope for rainwater drainage for a flat roof
If water is ponding on the roof, there is essentially no continuous slope to the edge.  It doesn't matter what the slope is if the water can't run off the roof.  How long it was there isn't really relevant in my opinion.

But if you want to measure the slope in a localized area, take a level - for convenience, say it's 1 foot long.  Put it on the roof and raise the low end until it reads level.  Now measure the height that you raised that end to make it level.  That's you're slope.

So if you lifted it 3/4", that would be 3/4" rise per foot of run.

If you had a 3' level and lifted it the same 3/4" to get level, you'd have 1/4" rise per foot of run.