well the 10-15 Percent rule is quite accurate and will give them healthy growth. Usually the size of prey if fine as long as it's not way too small or way too large, as long as they get it down easy, no worries.

It's not always easy getting perfectly sized prey for your snakes, so i usually just try and get 2 smaller prey or 1 perfectly sized one. Also your feeding schedule is perfect, just keep it up.

If it makes you feel any better i haven't been able to feed my larger snakes the right sized pretty due to a medium rat shortage. So i just feed 2 smalls.