The Boil Off page allows you to calculate how much loss you’ll experience as the result of boiling your wort. Essentially the end game is to find out how much liquid you have left in order to try to hit target volume and gravity. Using this page is straight forward except for most of the time people get tripped up on the “Evaporation Rate” and “Cooling Loss Percent” so I’ll explain.
Evaporation Rate This has to do directly with your kettle and boiling conditions. The way I figured out my rate is to boil 5 gallons of water at a set flame on my burner for an hour and measure what was left when it cooled. Keep in mind that setting your flame higher will cause this rate to go higher and different kettle sizes will cause this rate to vary.
Cooling Loss Percentage Has to do with the decrease in your water volume as it cools down from boiling. For the most part homebrewers are safe with 4% here. I’ve read it can vary upwards some and other tools have it set at 4.75% however most widely accepted as best I can tell is 4%.
To bottom line on the Boil Off page is that you’ll have to experiment with your rig to figure out an evaporation rate.