I would say 100ppm and some calmag would be fine although if you wanted a more accurate way to measure, you should put the right ratio of tap water to feed to make you cal/mag the right level relative to your feed strength.
Depending where you are, you could get a water quality report from your local water company. In the UK they have to supply you one for free on request so most companies put one on their website (you may have to hunt about for the PDF download that has the actual info in it)
Then you would know exactly how much of each chemical was in your water, so you would know how much to leave in to get the cal/mag ratio for your feed strength. You may find you do not need to buy any, just get the right amount of tap water left in your tank.
Here is a water report for Mayfair, London, UK (PDF). It shows the TDS (in us/cm) and amounts of each chemical (calcium, magnesium etc in mg/l) in the water, as well as any bacteria (if any) and other contaminants.
After getting the report for my area, I found myself trusting the water much more than when it was an unknown quantity because I knew how much to put back in to get enough Cal, Mag, and other micro nutrients.
I also found that they try to limit the mag in the water to 30mg/l or less as it is a useful nutrient for brewing but more than 30mg makes beer bitter TMYK