why are you running a pad and then jacking up the preamp? Pull the pad out and turn the pre down. With the mix pre as your front end you should be totally fine to cut foley with that rig assuming that you're in a very quiet room.

generally foley noise floors can be moderately forgiving since they'll be interplaying with the location audio from the dialogue track, but you do want to be careful not to have a bunch of noise ganging up on you across multiple tracks. Backpacks are also relatively loud things to cut so if you're experiencing too much noise I'd look either at your room or your gain structure.

also, to answer the question in the title - acceptable is what sounds good. Its an ear thing not a measurement thing, and it is highly dependent on context. If the backpack is on a kid running down a city street then you're going to be able to get away with a lot more noise than if the backpack is on a student in a quiet library.

I think as a matter of course and principle you'll want it to be as quiet as possible.

Think of it this way, the higher your noise floor the louder your sources have to be, or the closer you have to be to your object. You want the sound you're aiming for to be loud enough that its spectral (frequency) content is not being affected by the sound of the noise itself. This means that you could record something like a gunshot in a much louder environment than you could record the sound of turning a page in a book.

Gaining up will definitely bring up the source sound, but the noise is going to come up with it, so you want as much distance as possible (in terms of amplidtude) between the noise and the source. -50dB probably isn't too bad, but you're going to run into problems when you try to do delicate stuff like textures, closthes rustles, and things like that.

A decent studio noise floor would be less than 20 dB A spl.