I know a few people here in Europe who can make any set lowering springs for you.
But for BMW's he advised, front not lower then 35mm and rear not lower then 25mm.
They made a set for me 10% stiffer then OEM and the following lowering :
Front 28mm (1.10")
Rear 16mm (0.63")
With this set the front and rear gap is almost the same, maybe a difference of 1 or 2mm.
It is a little expensive but it is custom build.