I'm looking at options for new shocks and found some Fox shocks that fit the bill. Trouble is you buy the springs separately. How do I calculate exactly which spring will work best in my application?

Weigh each corner of your buggy on a bath scale to get empty weight. Then do the same with driver and passenger in the vehicle or equivalent weight in the seat. This will give you an idea of the spring weights required and how additional weight is distributed in your buggy. You may find that you need stiffer springs on the driver's side...

are you looking at front or rear shocks? I was thing about joyner rear shocks on mine

Pgh is right on. Thats how we measured the race car. Except we had racing scales that you set the whole car on. Except this was done to add weight depending on the track.