I did a Dr Pepper Butt this winter, and was definitely the best PP I have done.
We went to Memphis in May this summer, and did the tour of champions, they were doing butts the day we went. I did my best Walter Cronkite and asked as many questions as I good. All 3 of them injected, All 3 of them within 2 hours of it going in the smoker. Mushy was one of the reasons that they all said within 2 hours.
Changed my thinking quite a bit, I always injected the night before. Now I do my injection and other prep work, then go get the smoke rolling, so its usually 1 1/2 to 2 hours between inject and the smoker, did that with the turkey breast I did last week and they were fantastic.
I did over night with Mojo Crillio and pineapple in a pork loin, and there were pockets in the loin, where my injection technique wasn't perfect, that were little abscesses of mushy meat.
I like the 5% idea, definitely better than the normal "Inject as much as it will take" instruction you see with a lot of recipes.