jezus christ martin, use the bloody calculator in post #36.
sorry ... no, an 8 inch wheel requires an ET of 58 to stick out the same as a 7.5 inch wide wheel with ET45.
going to lie down now. exhausted.
I think you have over compensated Peter!.
Martin, try to think of it this way. If you have a rim, for argument, say it is the standard 7.5" wide, 68 offset. If you were to fit a 255 width tyre to that, the tyre would over hand the rim equally on the outside face and on the inside face. This is effectively what you do if you make the rim wider.
So the important bit for you to know if that for your car if offset numbers get bigger the wheel moves inboard. If they get smaller the wheel moves outboard. Conversely if the rim width numbers get bigger then the outer face moves outward (the inner face also moves inward so you always have to check for clearance there too)
So in your example of 45 offset on a 7.5" rim adding half an inch width, using Man Maths numbers of 12mm total, so 6mm per side, then the outer face has moved out 6mm and the inner face has moved in 6mm. To bring the outer face back to the same position you would (as I said above, bigger offset numbers bring the wheel inboard) so you would add that 6 to the 45 giving you 51mm. What this does however is reduce the clearance on the inside of the wheel by that half inch (12mm) that you added in width.
If you buy a set of race spec wheels the typical fitment would be 8" with a 53 offset. What this would mean from the difference of the standard 7.5 x 68 wheel would be the outer face would be the 15mm further out with the offset change and then a further 6mm for the width change so 21mm, which is about where your spacers were putting you and is about where your 7.5 x 45ET wheel is.