On the stock BBS, which are a +38, I run a custom 21mm spacer (giving a +17 overall offset) and it sits perfectly flush with my 255/40/18 Michelin PS2's (with it strapped down on the dyno the wheels tucked perfectly up into the wheel wells with zero rubbing).
So with that being said I would run a 22-23mm spacer and if you google Motor-Sport Tech they are who I buy all my spacers from for my car, my dads cars and all my friends (about 8 sets so far) and never had an issue with them and they are on some very high end cars ($100,000+ vehicles).
Also I dont know if your 235 35 18
is a typo but I am hoping it is because that is over 1" shorter than the OEM tires. If you are really stuck on the hole "skinnier is better" for winter and want a 235, go with a 235/40/18
so the over diameter is only 8mm different than OEM.
But on the hole "skinnier is better" theory I think its absolute bull shit and not true what so ever. To give you my own personal experience with this, Our R8 runs 295/30/19
SNOW tires and it is unstoppable in the snow. We just had a storm blow in 6"+ of snow and places where our new Tahoe with winter tires was having trouble, the R8 had zero issues at all even with low crawling speeds. So in my honest opinion here, stick with the stock 245/40/18 tires for winter and go with the 22-23mm spacers from Motor-Sport Tech (they call them wheel adapters) and call it a day..
Here is my idea of the perfect size winter tire. Thats a size 10.5 shoe....