Weigh snakes "dry," after they leave waste, preferably with poop and not just urine and urate. That will give you the most accurate weight. Snakes eat big meals and can take big poops. It can throw their weight off noticeably.
It's important for two reasons.
1. Appropriate sized prey (although you can use the sight method as well - with a BP, you don't want any prey item that leaves a noticeable lump. They are heavy built snakes, but not super stretchy.)
2. Are the sick?
Many BP's fast for periods of the year, usually winter. My BP Shayna fasts every winter for about 5 months and loses about 6-9% of her body weight. I do not worry unless she loses more than 10-15% and she always starts eating after about a 8-9% loss. It helps to keep track of weight loss during this period.
Additionally, if snakes are not gaining weight consistently when growing, that can point to health issues, such as internal parasites, etc.