Im the same as adam. Ive done a whole painting and didnt realise i was on 100% opacity and flow till i finished :|. I dont think i know anyone who could tell between a flow of 75 and 70
whenever i'm quickly scrolling through blending types to get some kind of random coolness on a new layer overlay, I almost always start with a fill of 75%. Seems to give the best overall impression of the various effects you might be able to get. Would probably be 66% if it wasn't for exclusion and difference.