Check out the review of the pedals here PowerTap P2 Power Meter Pedals In-Depth Review | DC Rainmaker.
That write up may lead you to the information you are searching for. There will always a margin of error when it comes to data collection so if one device is known to operate within a certain threshold and the other is relatively close to it and behaves similarly, then you could be looking at both of them being “good enough.” The difference between 70 and 76 could just be within the margin of error with one reading a little low and the other a little high or a combination of various levels.
Crank length won’t matter to the pedals though.