500ml Trace (MgSO4) Solution

I am not sure what you are trying, but if you want to add Epsom salt to the water add whatever amount you want (like 1/8 tsp or 1/4 tsp or whatever) to as large or as small an amount of water you want, stir it briefly and dump it in the tank. You could even dump in the dose dry.

1) How much 'active ingredient' do you want to add to the tank?
2) Add it.

The PPS Pro recipe calls for 41g MgSO4*7H2O dissolved in 1L for a dosage of 0.1 ppm Mg when added at a volume of 1 mL/10 gallons. Therefore, your old recipe is correct if that is your targeted daily dosage.

96g MgSO4*7H20 dissolved in 500 mL will obviously result in a dosage ~0.5 ppm ppm Mg when added at a volume of 1 mL/10 gallons.