Listen to @blacklab and @bobbrews. The evaporation rate depends on the surface area of your brew kettle, the rate of boil (e.g., rolling boil), and to a lesser extent, the altitude. A percentage can be accurate if you always boil the same volume in the same kettle at the same rate, but as soon as you change one of these variables that percentage will be wrong.

If your kettle is tall and thin, it will generally have lower evaporation rate than a short and wide kettle. If you use a simmering boil, your evaporation rate will be much lower than a vigorous rolling boil. It doesn't matter how much you are boiling, because the boiling rate is the same (as judged from the surface - simmer, rolling, etc) and the surface area for the pot is the same.

If all we cared about is minimizing the evaporation rate (which would save water and energy), we might choose a tall, think kettle and simmer for an hour. But beer quality usually trumps these concerns, so the choice of brewpot and boiling rate is not so straightforward.