I believe the increase is going to depend on the buffering capability of your water (I could be wrong on that particular point), but I would say anywhere from 2-4 tenths up to a full point. Such as 7.2 up to 7.4 or 8.4. A popular rift lake recipe is 1 tsp baking soda, 1 tsp aquarium salt and 1 tablespoon epsom salt for 5 gallons. You will either have to continue to add with each water change or provide some type of buffer to hold the pH where you need it (coral rock works well and driftwood)
Someone with more experience may want to chime in though if I am off a bit.