When measuring fired cases using a headspace guage, you should do this with the used primer removed.
As mentioned,a Lee universal decapping die is ideal for this. No lube is necessary to use the die.

Measure ten or twelve cases and take the average dimension, then set your F/L sizing die to resize to that dimension.
Your case shoulder 'bump' should not exceed .002"...it's unlikely to anyway if you're measuring & setting your die this way.

Daryls method works just fine, it's a trial & error method not involving additional tools & guages.
In any event, full length resizing only minimally working the brass, will certainly ensure easy chamering and extended case life. In my view way better than neck sizing also more consistent.