It comes down to knowing your boil off rate. If you can keep it consistent, it's easy to figure out. For example; I have exactly a 1 gallon per hour boil off rate in my set up. I always do a 90 minute boil and I start with 7 gallons and end with 5.5 gallons. Every time. If I'm going to shoot for a 1060 OG, I would calculate my gravity units in my wort by using the last two digits in the specific gravity. So 60 × 5.5 = 330 points. So no matter what the volume of liquid in my kettle, it needs to contain 330 points. So to figure my preboil gravity, I divide that 330 by 7 gallons to get a preboil of about 47. So when I lauter, I'm looking for 1047. I also lauter 9 gallons into 3 different pots so that I can blend them to get exactly gravity that I want, in this example 1047. I use those same calculations to figure the gravity points of all three pots. Ever since I started doing this, I've always hit my target.

It's a lot more simple than I just made it seem, I promise.