I moved bottom plate close to wall ~6mm (total distance 32mm) and still could put upper bracket with no problem. I think that I can assure you that 33mm from wall is safe. So if the edge of CPU mount is at least 14mm from Motherboard edge – you are OK. (It is 19mm from the Motherboard edge on H81M-ITX)
I hope that my info and pictures are helpful.
