Brew cat is right every system is different. Your boil off rate will be a big factor, and even the amount of hops in your recipe. But I think you'd be on the right track looking to get 5 gallons into the bottling bucket or keg. If you have a refractometer, you can boil for about an hour then cut the heat and take a quick reading. If you're at your target gravity then you're good to go. Higher gravity than you want, just have a little water handy and add. Too low gravity, then just turn the heat back up and boil a little more. Of course this method can screw with hop utilization, so if it's a hop forward beer this method might not be great. But on the first batch of any recipe, I just use a calculator for total volume I need going into the BK based on my boiloff rate. Then in the end, if I get 5 gallons bottled, great. If not, I adjust sparge up or down next time to make it right. And if you're getting decent efficiency you should get a good beer. I'd be more concerned with the quality of the final product rather than worrying too much about a quart or two of volume.