It depends upon how much mineral content is in your tap water to begin with. You need a TDS/EC meter to get it right. If you live in the Missouri river basin (limestone substrate) the water is already loaded to the gills with calcium and the answer is ZERO lol. If you live in New Hampshire (the granite state), the substrate is insoluble silica based igneous rock and you'd need calmag. Same with RO water- no minerals; need CalMag.
I'd test my tap water and see how much you need to add to hit 150ppm as CaCO3 equivalents.
Measure out a 20L pail. Fill a 100 mL graduated cylinder with CalMag. Drizzle it in while mixing, stopping to test occasionally, and when you hit your 150ppm target, write down the level remaining in the cylinder. Subtract that from 100. That's how much you used. Now divide that by 20 and the result is the mL/L required to condition your water. This is called a titration.
It's very easy and you should do it about 4x per year if your tap water comes from a surface source affected by snowmelt or rainy / dry seasonal cycles. Water from wells/ aquifers really doesn't drift that much and can just be checked annually.