I’ve been using info from this and other similar threads as I went with 19x9 +34 square setup on a 2018 M3 LR. I was initially thinking of going with 245/40/19 but I bit the bullet and purchased one 255/40/19 Michelin AS4 tire to do a test fit. There is definitely some additional room to clear wider section/tread widths, and some ~7mm to go in terms of tire radius before touching the knuckle. On full turn lock, there is about 10mm before rubbing between outside shoulder of tire and back of wheel well.
For what it’s worth, another data point for non-performance M3
19x9 +34 squared setup
255/40/19 Michelin Pilot Sport All Season 4