I found this from an old x-drive manual. Though the original setup has the rears 0.3% smaller, I would be ok with starting 0.7% bigger as the rears tend to wear faster anyhow if you use your throttle heavy (on our 50i, it's very noticable).
Also why I would prob switch to a square setup.
I'm prob going to a square 275/50/18.
E70 M has up to 285 fronts so I'm not concerned about 275s clearing body/suspension.
You can see the size I'm aiming for is nearly identical to the biggest tire BMW puts on the front.
In my case by going from 255 > 275 up front I will likely bias the car a little more towards oversteer, but likely not enough to actually make it oversteer. Something I've only seen a couple times on slippery surfaces.