Paul, this is more complicated than you think or most of us even know. First and foremost, it does not matter what the pH is of your well water. Based on discussions with Randy, there is no correlation between two different pH levels being added together. So if you have 100g of pH 7.0 and combine it with 100g of pH 8.0, you will not end up with 200g of pH 7.5. It just doesn't work that way, but I can't give you the scientific reasons why that is.
Once water has gone through the RO/DI process, it should essentially be pH nuetral anyway. Then you mix it with salt, which typically raises the pH, but it is not a consistant result. It all depends how much you aerate it, how long you age it, what the ambient conditions are etc.
The simple reason why reef keepers use kalk for top-off, like I do, is to keep the tank pH up when dosing CO2 into our Ca reactors. The CO2 tends to lower tank pH and as demand for Ca grows, we can run into low pH issues. Using the kalk keeps the pH up, and there are various ways to control it.
I use multiple controls for my top-off. I use a LiterMeter III for top-off and have it set to a little more than what my tank's needs are. I have a float switch for water level control, so it won't go on unless the water level lowers due to evaporation. Then I use tank pH to control it so if the pH get above 8.35, power is cut to the LiterMeter.
You also must be very careful about how your kalk is stirred. The liquid in the reactor should remain clear. I have a Barr stirrer and so I also have that set on a timer to stir twice/day, and during that time and for the following hour, I also cut the power to the LiterMeter. That way I am NEVER dosing kalk fines into the tank. That can cause serious problems!