A given mash system has a certain amount of dead space from which liquid will not drain and will retain a certain amount of liquid in the grain. You need to get a sense of what that is. There are estimates for how much water grain retains but there is some disagreement since the total retention is some combination of grain retention and dead space. While you can try to use an expected retention rate in qts per pound, it won't be perfectly accurate, since it doesn't describe the dead space in your tun.
What is important to know is how much liquid your system will retain for a given weight of grain. One way to do this is to do a handful of batches with the same total grist weight, and the same target volume. What you do is you make a graduated rod for measuring the volume in your pot, and after you drain the first batch from the mash tun, you determine how much you collected and subtract it from your strike water volume. You then need to add a batch-sparge volume that will make up the difference between what you collected and your target boil volume.
So for instance:
I mash 10lbs of grain with 15qts of water. My target boil volume is 26qts (6.5g).
I drain the first runnings from the mash tun and collect 10qts. I can assume that there is 5qts left in the tun. I also know that I need 16qts more for my target volume, so I batch sparge with that much. Next time I make a 10lb batch aiming for a 6.5 boil volume, I can prepare the 16qts instead of having to overestimate the amount I will need. If I'm aiming for a 5g boil, I can just prepare 10qts sparge water. The important thing to know is that you'll need (in my case) 5qts more than boil volume for 10lbs of grain. You can split that between mash and sparge water however you see fit.
Of course, if you do a 15lb batch, you'll have more liquid retained, but if you measure the runnings, you can determine how much you should add for the batch sparge. If you keep track of how much water seems to be retained by a given weight of grain, you can use that information in the future.
Hope that helps!