This is exactly one of the cases where … imho … AimPoint really stands out!
In my post above where I listed two examples from my last 9 hole round, example #1 was just this situation - and getting an AimPoint slope and speed read gave me what I needed to nail that putt!
To your other question - flat vs 1% - as @davep043 mentioned… with some practice, absolutely yes!
Similarly 1% vs 2%, 2% vs 3%, etc etc. I have not seen the video, but in my clinic the instructor showed us his preferred method for getting a reading (and sorry but really cannot describe it well in words … it’s really one of those “show me” kinda things).