15x8's on a mk2 4 door

ok guys ive got 13x8's to fit now onto my next challenge, looking at trying to get a set of 15x8's under std arches on a mk2 4 door. 
now before anyone says anything i plan to run silly skinny stretched tyres something in the 175/55r15 range, so keep that in mind.
 
what i need to know is what offset ill need to aim towards to get them sitting inside, or flush with the arch.
 
 
and secondly if 8's are a no go, have little offset can i get on a set of 7's, car currently fits 15x7 12P with easy with big sidewalled tyres, thinking in the range of 0 to -5 with some tyre stretch???
 
thanks guys

What you need to do is measure the distance from the hub face to the wheel arch edge and work backwards.  For instance, I measured mine at 95mm from hub to the the inside edge of the arch (front and rear), as I don't want to have to roll my guards.  If you want to go to the outside edge, the dimension would be larger; about 12mm for a total of 107mm.  From there I used a spreadsheet to calculate the distance to both the wheel face (bearing in mind that alloy wheels are usually an extra inch wider overall) and tire face.
 
For example, using 95mm and an 8" wheel the offset would have to be 19p, giving a 133mm backspace, which I suspect is too large (i.e. it may hit the strut).  If you went to the outside edge of the guard then those figures change to 7p giving a backspace of 121mm.  
 
Now for the disclaimer - I made the spreadsheet to assist my wheel size selection, but still verified my final decision by checking out a car that used the same wheel and tyre sizes as the spreadsheet predicted.  As such I can't make any guarantees that the above figures will be right for you as (1) your vehicle measurements may be different to mine, and (2) you appear to be pushing the limit of what is possible without using arches, and (3) despite my best efforts, my spreadsheet could have errors in it.  As such you should use the figures I quoted as point of comparison with your own measurements and calculations, and ultimately make your own decision about width, offset etc.
 
Good luck!