Backspacing question?

[RobertE]

I'm wondering what the correct backspacing would be for a 15x7 wheel?

I've seen people fit 15x7 under the stock body work and I'm hoping I can get by without rolling the fenders.

Thanks once again!

Edit: It might help to include what type of car it is. 79 mgb

[ex-tyke]

Keep the same offset as the stock MGB wheel (22mm)
The real issue is the width of the axle that you are using (if non-std MGB) and if using the std B axle then you have to cope with the documented issue of axle bias to the left side of the vehicle (about 3/8'). Given a perfectly centred MG width axle and 22mm offset wheels you should be able to use a 215 width tire with no interference.
Just as a bit of history on my conversion (Panasport 15x7 w/ 22mm offset, 225/50-15 tires, Ford axle made to MGB width, and with spring mounts welded 3/16" offset (to compensate for the afforementioned axle bias),  With this combination, I still had to remove most of the flange on both wheel archs and also provided additional clearance by a subtle rework to the contour of the fender and by slightly flattening the right side bump stop. In retrospect, I should have gone a little further with the spring mount weld location as the left outer tire still rubs but not the right tire.