If you put her into a snake bag (cloth bag -- pillow case works well) or into a plastic container (small storage tub), weighing is much easier.
Interestingly, studies on snake (python) digestion seem to tend to use meals of 25% snake mass (source, source). I think that's silly big, but it shows that such a meal is within a normal range (and wild snakes -- which ours are not, and shouldn't be treated as such -- are known to eat some truly gigantic prey items). I personally would lean toward increasing meal frequency a bit, and/or hunting up some rats that are just a little larger than the 100g ones. I agree that a regurge is to be avoided.
I do have more than a couple snakes (not BPs) that act hungry all the time and would eat themselves into a very early death if allowed. I have no problem with moderating their intake based on body conformation and what I think is a decently small prey item, and they can just think they're hungry (which can be an artifact of captivity, at least in some species). I tend to feed my snakes when they tell me they're hungry, unless and until that becomes apparently harmful and then I take over the scheduling.