Changing body position can have a significant effect on BPEL measurements. Erections are sometimes fuller while sitting or standing than when lying down. Position also changes the orientation of the pelvis which can alter angles. Some guys might just be measuring to compare themselves to other men or the mean. Other guys into PE are trying to establish a baseline measurement and then comparing subsequent measurements at intervals to evaluate gains. In both cases, you will want to use whatever position you are able to have a maximum erection, and then stick with that same position every time you measure. In establishing a baseline you definitely will want to measure routinely for a few weeks before starting any PE techniques to make sure that you are able to replicate your maximum as a routine. Erection quality is the key. Without a reliable baseline you might get false positive gains or false negative gains in the future. It happens all the time.