I have to take my bike in a small elevator, so sometimes I need to hold the bike up on just the rear wheel (front tire is in the air at about head height).

I'd like to cut my fender down a bit to allow this function - where can I find a diagram that will show me how much I can cut off safely and still avoid a trail on my back. I don't ride in a group, so keeping people behind me dry is not a priority. Thanks!