A scale is one of the most important husbandry items IMO, whether you're planning to breed or not.
But to answer your question, I don't know anyone who would feed a bp two medium rats in a row. Adults do not need to be fed meals as large and frequent as growing snakes. I have heard many people, breeders included, who don't ever feed anything larger than a small rat to an adult bp. You should be fine on one small rat every 7 days...
EDIT: The general guideline if you really are against buying a scale is to feed one prey item approximately as big around as the thickest part of your bp.
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