You can do a test boil to get an idea of what your boil off will be. But you cannot use that as a percentage to calculate the loss for other size boils. If all other conditions are the same, your volume loss will be similar (not identical) regardless of the size of your boil.
As wilserbrewer said... best bet is to make beer and learn how your system reacts.
Personally, I would prefer to undershoot the volume and slowly add water in the last 15 minutes than to overshoot. If you extend the boil time to reduce the volume, it will impact your hop utilization. Of course adding water to the end of the boil tends to kill the boil, so starting with the correct volume is preferred.