One thing I've learned over the years is to feed them a prey item that looks slightly too big. They can handle a little more than you think. I have never weighed a prey item, I judge solely by look. Eventually you'll be able to do the same and not stress about it.
Here is a picture I took for size comparison. That snake was about 75 grams and already has a rat pup that size in his belly. I only fed him one and took this one out after the picture.
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