How do I calculate amount of Gypsum needed in my boil... preboil volume?

As thread states... preboil volume or batch size? I assume it's preboil but i would like to double check.
thanks

It depends on what you are trying to do. Gypsum does two things: it provides calcium which lowers mash pH and it provides sulfate which synergizes with hops to modify and amplify how they are perceived. Thus if pH control is the goal it is definitely pre-mash concentration that you are concerned with but if it is hops perception that is of concern then it is kettle.

In most cases you either have a mineral profile you are trying to match e.g. Burton: x mg/L sulfate, y mg/L calcium etc. or are following a general recommendation e.g. 5 grams gypsum per gallon. In either case you are preparing water that resembles what comes into a commercial brewery (though it may have been defunct 100 years). So it is pre-boil and, in fact, pre-mash.

It is far and away simpler to treat the entire volume of water to be used on brew day the same way but if you are doing something sophisticated there may be times when you want to add salts (or more probably, acid) to the kettle that you did not add to the mash. IMO these are techniques for advanced brewers.

Right now just trying to match tasty's profile, adding it post mash pre boil. Couldn't remember if I used preboil volume or batch volume

You should be adding minerals on a pre-boil basis. Brewers don't go around checking their post-boil ionic concentrations, but they do know what the pre-boil and/or tap water concentrations are. In addition, there are many reactions that occur in the mash and boil that can either increase or decrease the concentrations of the various ions.