Just to explain a bit. The offset can be negative, positive or zero.
Positive is what is found on most front wheel drive cars. In this case the hub mounting surface (where the wheel bolts up to), is towards the outside of the wheel. So, effectively more of the rim and tyre are towards the centre line of the car. The offset is a critical part of the steering geometry which determines what happens when you swerve and/or brake.
You can physically vary it but until you find yourself in an emergency situation you can't guarantee the car will do what it's designed to do.
I imagine a few points like you are suggesting would be OK, but the fact that they are 7" wide, not the standard 6" of factory 16" wheels may have some bearing on it. They may need a bit less than the 50 in order to avoid rubbing on the sub frame on full lock.
I had to Google what ET is. According to the reply, "The offset is described by the term ET, which is from the German word 'Einpresstiefe' translated as 'insertion depth'"
We just say "offset"
So, in short, I can't give you a definitive answer.
