During my typical 60 minute boil I boil off around 2 gallons, but I have a short and wide pot with a diameter of 18". A typical turkey fryer has a much smaller diameter, so on this first batch if I were you, I'd assume a boil off of 1 gallon and shoot for a starting volume of 6.25 gallons. Check at 30 minutes into the boil and see where you are. If you assumed you are going to boil off 1 gallon in 60 minutes, then at 30 minutes you should have boiled off 1/2 gallon, if you have boiled off more than that, make adjustments like turning your flame down a little or even topping off with a little water. Remember, you can stay on your hop schedule and add water, but you can't stay on the hop scnkshedule and boil longer.
Once you've done it a few times this won't be an issue. I know temperature, humidity, intensity of boil and the wort composition all have an effect, but I stick with 2 gallons at this point and don't have problems anymore. Good luck!