Adding a ball valve to a boil pot - how high?

If by "dip tube" you mean pick up for a false bottom then I would just measure it. If you don't have it yet, it can always be cut to fit...my ball valve is ~ 3" off the bottom...YMMV depending on your kettle.

I think I would stay with a bazooka screen if it's for a boil kettle and you use a lot of hops.

 

Mine is 1.25 inches from bottom and is working fine.

 

I have done this a couple of times. My advice is to put the valve as low as possible while also keeping it high enough so that the o-ring that makes the seal is above where the pot starts to turn under towards the bottom. If you have room in your pot you can lower the bazooka screen to the bottom of the pot with some fittings, as I have done in the following picture.



These copper fittings are just friction fit together. (To get some friction I used some pliers to make the Cu tubing slightly out of round.) I find that the bazooka screen works great if one uses loose whole hops. If I were to use pellet hops with this setup, I would contain them in hop bags. Otherwise I believe that they would clog the screen.