Water leaves the tire on a tangent (although it looks radial). So, if you cut the fender to where it jut barely allows the rear tire to touch the ground when vertical, the spray leaving at that point would be vertical and not wet you unless there were a strong tailwind.

You could buy more forgiveness to put the bike vertical by attaching a stiff but flexible, rubber or plastic extension 4 or 5 inches long. That would allow the combined fender to extend beyond 3 o'clock (when the bike is on the road) and yet allow standing on the flap without damage.

BITD when I had a fendered bike and rode elevators with it, used a stiff rear brace that could handle the weight of the bike then stood the bike up on my foot and "walked" it into the elevator, and out again at my floor.