you have to experement on the actual dosage. trial and error. start slow as kalk will raise the alk
...Not really. If you know your system volume, raising your Ca or Mg levels from XXX to YYY is a simple matter of math, so long as your CO3 levels aren't outrageously high -- and if your mag's that low, they won't be.
Raise your Mg first, then raise your Ca, then correct for Carbonate.
If your Mg and Ca levels are OK, Kalk will generally only raise your Alk to slightly-above-normal levels. Once it gets above that point, the incoming kalk will precipitate out. (Same thing goes if you're raising the Ca levels too high.) That's one of the nice things about kalk; so long as your levels start out OK it's pretty hard to knock them wildly out of balance, in my experience.
Additionally, your carbonate levels are directly related to your pH so long as the Mg level doesn't change; kalk supplementation can help maintain a more constant pH, and thus alk, during the night when corals are doing all their skeleton-building.
So, if you adjust your Mg levels to the point where pH 8.4 or 8.5 gives you a good alk reading, and set your top-off to only run to 8.4 or 8.5, you won't overrun that number, though if your evaporation rate is low you may need to supplement to raise your carbonate/pH.