Thanks. Yeah, I've seen that and the guide at Bimmerworld as well.
https://www.bimmerworld.com/About-Us/Wheel-Info-Tech/
Have also been reading through the threads here. Should have been bookmarking them as I go. Have a few;
https://www.2addicts.com/forums/show....php?t=1597122
https://www.2addicts.com/forums/show....php?t=1602402
https://www.2addicts.com/forums/show....php?t=1804723
Everything seems ridiculously tight, a couple mm of offset or even different makes of the same size tire can be make or break. I picked a car with the stock 225 all seasons with the intention of getting aftermarket summer wheels, but I should have researched this more at the beginning - when I was deciding between an M240i or an M340i it might have steered me in the other direction, but it is what it is. Worst case I can always go with the factory staggered setup I suppose.
So from going through this again, I'm probably best to go with 8" wheels, not ideal for 245s but doable, and it looks like 40s rather than 35s aspect ratio as well, with something around 38-40 offset, or spacers in the front. I do want to stick with 18s, even though 17s are frequently recommended for square setup.
Of course after choosing a size, then I have to find wheels I like in that size. This is my 4th BMW now, and really the only time I've come to question German engineering.