Let me rephrase, 1.5x max. I always went by weight on my small snakes. Never had a problem personally have never had a regurgitate. I mean do what works for you. For me, like I said, I weighed my food out for my pied girl and she went from a 102g 12" baby to now almost 4 months later, she is 3ft and over 500g(haven't weighed her in awhile). Ball pythons aren't known for regurgitating like I said but a lot of BCC will regurgitate if fed too big of an item, thus why you generally feed them smaller prey. Same with BCI, they will eat pretty much anything, anytime, anywhere. And yes once ball pythons hit 500g, you go by eyeballing the food size and not by weight anymore but as a baby, a lot of new people say "oh no way my baby can eat a ray pup etc etc" but like I said, you would be surprised what they can eat and feeding them something like a pinkie or fuzzy is really just underfeeding them at their most crucial growth time, the first year. Again do what works for you but I am telling you what has worked for me with many many snakes.
I take that back, I had one regurgitate about 20 years ago and that from one of my Savannah Monitors which ate too big of a rat but that was only one monitor out of 3 different ones ive had. Never had any of my snakes regurgitate including my first ball python I ever had about 30 years ago when it was glass tanks or home made plywood/stud framed cages and heat lamps with mesh cages around them.