It’s a hard one. Flaccid size changes a lot. Probably the easiest way is to take many measurements and then take an average. This has the advantage of capturing the range of possibilities. Taking a maximal measurement and using that has the disadvantage that you are potentially taking a measurement that is part way toward erect, which is maybe more of a moving target.
You could potentially take measurements at fixed points in time during the day, at parts of your day when each day is similar and average those or pick an at rest time where you know you haven’t been doing anything that could affect you positively or negatively.
You can ignore any flaccid measurement taken after PE as a flaccid measurement though it might have use in telling you how effective your PE was. Experiences seem to vary though.