Hard to get an "exact" proper weight... unless you wait it out and wait till you know they've pooped everything out. I just weigh whenever I have the opportunity. But, generally speaking, I don't weight right after a meal. I'd wait a few days at least. As your snake gets larger, the minor grams from a meal are less relevant anyhow.
A good experiment would be weighing your snake right before a meal. Then after feeding give the snake a couple days to digest. Then weigh every day for like the next week. I guess just to see how much the weigh adjusts as the food is digested.
Maybe I will try that.