Mine are not on lactulose either, my vet also said that it should only be used in more advanced cases and that mine didnt warrant it. My vet did recommend the Denamarin but mine threw it up everyday, so the vet recommended we take them off, she said not all dogs can tolerate it.
The usual recommendation you see, is nothing higher than 18% with symptoms and nothing higher than 22% otherwise. However, my vet and I agreed that with my dogs being so young and healthy (besides the high BAT's), such a low protein food was not necessary for them if I wanted to do something higher.
My dogs were on a 24% protein food (Innova small bites) and they did fine, I switched only because of a problem with the company (Natura) nothing else. Mine are doing very well on the Natural Balance Organic,
I also add some chicken and potato canned food because one of my little brats is picky and won't eat only dry food 
This does increase the protein level though.
Honestly, my one dog was on a food that is close to 40% protein before I found out and he had zero issues or symptoms on it. I wouldn't stick with it now that I know about the MVD though.
Basically, I focus most importantly on the TYPE of protein, I stick with chicken and fish based foods, and will add other proteins like cottage cheese or egg on my own sometimes.
I was not happy with the ingredients in any of the prescription foods which is why I chose to find something else.