The 4th was 4 oz.short. So even when I did the exact same procedure I got different results. I used a kitchen scale to weigh a brand new one (1 lb 14oz), empty one 13oz & then again after filling. It is very easy & it paid for itself this first time. I refilled them for less than .40 cents each.
https://www.amazon.com/Mr-Heater-Propane-Refill-Adapter/dp/B000AMC5WO