Well lets do some math here shall we.
Say you are running an 18x8.5 +12 in the rear, now that is perfectly flush with just a roll and 1* negative camber.
Now if you add the 50mm over fender you would need that 18x8.5" wheel to have an offset of +12mm-50mm or -48 at a mere -1* offset which is pretty abnormal. Most nicer cars run -2* to -4*.
Now 1 inch = 2.54cm or 25.4mm so if you want a bigger wheel say a 18x10 you are adding 1.5" to the wheel but that is split and added to each side so in your fitment calculation you are adding .75" or 19mm. So an 18x10 will stick out 19mm more than the 18x8.5 . Following me? meaning for the 18x10 -29 would fit.
You can get to those sizes using spacers or try and find a wheel with that dimension.
The car below is running 50mm overs