In reply to both your private message and above post,
1ppm CaCl2 = .9090ppm GH = .0509 dGH
1ppm MgSO4.7H20* = .2425ppm GH = .0136 dGH
*epsom salts, this is what you will be working with
Your tank is fine right now; somewhere around 3.7ppm Mg (a touch low) and dead on 20ppm of Ca.
Good luck overdosing Ca/Mg. You could dose 4x that and you'd have some people's tap water. Your soft water plants may not be happy, but a 75% WC would fix everything with the tap water you have. In short, most of us envy your ridiculously soft tap water
For Ca/Mg, just work with EI and dosing; your Ca/Mg is unlikely to be higher than twice what you're dosing (still safe ranges) if you're doing EI. Some hard water plants can mess with things if you've got CaCO3 in the water, but you don't have enough to end up with that issue.
CaCl2 does not play nice with MgSO4 in my experience. Everyone says it should work fine; I end up with nasty precipitates no matter what method I use. Maybe it's forming CaSO4 or something.
For the MgSO4.7H2O solution, add 25.35g (or as close as you can measure to that) to half your volume of deionized water (125ml) then top up to 250ml. Stir for a while. Dose 1ml for every 2L of solution to get 5ppm Mg in your column.
For CaCl2 (odds are you have anhydrous, so there's no .nH2O to worry about. If it isn't, let me know) mix 27.69g the same way as above, dosed the same method, for 20ppm Ca.
In terms of testing for GH, you should make up a calibration solution. Mix up 5.44g of CaCl2 in 100ml of DI H2O. Every Ml you add to 1L of DI H2O will raise the water 1GH. If you test 2-3 GH in the column, test and see what 2-3 GH looks like on the calibration solution.
Careful with the CaCl2; wear gloves. It'll burn around your nails if you don't.
If you're feeling lazy at any time, you can pretty much just dose equal weights epsom salts to calcium chloride; 4.5g of each in 80L will get you 20ppm Ca, 5.46ppm Mg. You can also use the CaCl2 fert solution as a calibration solution; it should only be about 1.8% different in the results it gives.
For my own future reference on this thread, and maybe your own curiosity, here are the formulas I used to calculate things. They're MS Excel formated:
CaCl2 (Anhydrous) Solution:
GH Calibration Solution: