Amount of boil off depends mostly on the vigor of your boil and the shape of your kettle. Applying a lot of heat to the kettle so you have constant mini volcanoes will have a higher boil off rate than when only half of the surface is kept at a rolling boil.
A larger diameter kettle will have a greater boil off rate than a narrower kettle. This is a function of the surface area of the wort.
Best way to determine your boil off rate is to boil water for the same amount of time you would boil your wort and measure the volume remaining.