I think the 10 - 15 percent rule still serves a good purpose, especially for new Ball Python owners. For someone new to reptiles and even rats for this matter, this rule is a nice baseline to go by. For me, being a new owner when I picked up my normal python, I didn't want to "eyeball" the rat or guess if it was the right size, I wanted to make sure I gave it exactly the size he needed. (just me being ocd) I didn't take my scale to my local pet store, but I use to ask them to weigh them before I bought the rat to make sure they were the right size.
Having two Ball Pythons, for a little over a year now, I'm much more comfortable visually picking out the appropriate size food for my guys. Only reason I even weigh the rats now is just for my feeding records. I think if the 10 - 15 rule didn't exist, we would see a lot more threads asking, "is this rat ok for my snake".
The 10-15 general rule of thumb, for me, was really helpful when I first entered this hobby.
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