Out of curiousity, what size is the rear wheel? If it's a 18" then I wouldn't go taller than the 34/11 ... if it's the 17" then the 33/11 is fine. The stock FD on my '91 R100GS is a 33/11 with the 17" wheel. The Dobber (R100) currently has a 18" rear wheel, and the 37/11 ... and it's too short. On loose dirt, it's difficult to not spin the tire. I'll put a 32/10 on it (I have one on the shelf), and I think it will be a fine medium for road and dirt.
I have to say though, the 18" wheel + the 37/11 ... I can go 80mph @ 5,000rpm. That really is a sweet spot.