Just do it all in one day. You can usually tell within a few reps if you'll hit somewhere in the ballpark of 15. Basically the first few reps should feel easy. Just go til you hit failure, and if it's anywhere from 10-15, call the next lighter increment your 15RM. If it's anywhere from 15-20, that's fine. There's no need to be super-precise with the 15RM's.
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