Accuracy
The reason for plus or minus measuremnets is poundage and string stretch.
Tune your bow, when it's shoot the best it can, then take those measurements. Write them down in your bow log, those are more important than any other. Keep those measurements dead on.
Axle to axle measurements are hooking over the axle and measuring to the front of the other axle, hense center to center.
I allow no more than 1/16th difference in a tune measurements, most of the time it's no more than the thickness of the line on the tapemeasure.
Have a good bowhunt