Define "rat pup". There seems to be a LOT of variation in size of what people consider a rat pup. Some call pinks/fuzzies pups (common among pet shops trying to rip you off), but breeders generally consider pups to be that stage between fuzzies and weaned where their eyes are open and they are mobile, but not quite ready to leave mom. Pups are about the size of an adult mouse. Unless your ball is fresh out of the egg, a mouse hopper is probably a small meal for it and is why it accepts three in a row. It is amazing the meals they can eat. Most people new to snakes are amazed when they see how big an appropriate sized meal is and how they can eat it with very little effort.
As the folks above me have said, invest in a cheap digital food processing scale (Wal-Mart has them for about $12). Then just follow the 10-15% body weight theory every 5-7 days and you'll be all set.
As a final note, don't believe everything the pet store employee tells you. Some may or may not be correct, but the fact is they generally make minimum wage (that isn't enough incentive to learn for most), have no real life experience with reptiles other than maybe a company provided care sheet (which are generally very inaccurate!) and they are taught to try and up sell everything to make the company more money. Most of the folks here have been keeping and/or breeding reptiles for many years and they know what they are talking about and are very passionate about passing along the knowledge.![]()