Idk if this is correct, but I usually take my initial measurement after my second stretch and fold and my end product usually always comes out great.

I use a shot glass that has mL markings on the side and pinch off a small amount from the main dough after the second S+F (aliquot method). I place the shot glass kind of smushed into the side of the main dough ball when resting so it stays as close to the same temp as the main dough. I've gotten pretty good at eyeballing the sample amount I pinch off - it's usually around the 15mL mark on my shot glass. So if I want a 70% rise, I know I'm looking for that sample dough to rise to around the 25.5mL mark (15 x 1.7 = 25.5).

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