I'd go with a couple of scoops and go from there. If your nilsen reactor is made from clear plastic you should be able to see when the majority of it is gone. A general rule of thumb is that about 1.5 tsp of kalkwasser will dissolve per gallon of water so that might help you judge how much kalk you need per week.
I always seem to have some residue precipitate on the bottom of the reactor, even after it's exhausted. I can tell I need a recharge by monitoring the tank pH. It starts dropping when the reactor is out of ammo.
In addition to Liquid and ChriRD if you have a PH monitor (which is a very good item to have) the output from the reactor should be a PH 12+ for saturated kalk. If not then its time to "recharge" the reactor.
For the record I evap 1.67gpd X 7days = 11.69g per week
I use 1tblsp Kalk per gallon so I use 11.69(12)tblsp of kalk in my reactor for a weeks charge. Usually by the end of the week my reactor PH is down to ~9. So I know this method works for me.
The reason I dont add more and go longer than a week is a long story. In a nutshell I use well water that is high in bicarbonates that kills my reactor (PH <10). This will be remedied when I finally get my RO/DI unit