I work with underfed rescues often, and I generally do 10% of their body weight once a week. I think you might be ok doing something like 5% twice a week if you can only get mice.

It's amazing how much better a feeding response can be when a snake is housed in a proper cage, with the correct temps, and is generally feeling ok. I've had rescues that didn't really have anything wrong with them other than super skinny, usually with a stuck shed and/or mites, that the owner claimed refused to eat for months, yet I get them set up properly and get the shed off and in a week have them eating F/T with no issues whatsoever.