+1 to the above. Add a measured amount of water to your kettle, boil for 60 minutes, and measure what's left. Then when you brew, add the amount that boils off (your pot's evaporation rate) to your target volume and you'll know how much wort to collect - ie., say you boil off 1.2 gallons in that hour and want to be left with 5 gallons of wort post boil, then plan to collect 6.2 gallons of wort to be boiled for 60 minutes. Obviously if you're doing a 90 minute boil, you'd need to collect more wort to account for a longer boil.
EDIT - Steve's right that environmental conditions can change your boil off rate, and also there is ~ 4% expansion at boiling temps as opposed to cooled volume but this gets you in the ballpark.