system
Given the following post, respond as if you're commenting on Reddit:
human
[My Hero Academia] How does Midoriya calculate what percentage of One For All he’s using?

And for that matter how can All Might deduce how much power of One For All is being used?

Is it similar to how a person lifts weights or jogs for a long time?
gpt
I don't think it's a literal calculation of the amount of OFA applied, it's more of an estimation of how much effort he feels he's putting into it.  
Like he isn't literally putting 1,000,000%  into it when he does his 1,000,000% Smash, he's throwing his perceived limits out the window and putting everything into it. 

All Might is a good judge of it because he's in the unique position of having gone through almost exactly what Deku's going through, going from a quirkless weak kid to having one of the most powerful quirks in the world.