No problem. I try to help if I can.

What exactly is a grain per gallon?

Grains per gallon is just another of the many ways to express water hardness. There's GH, ppm CaCO3, now gpg. I know, it's confusing at times.

So what assumptions are we going on here for this equation to work? Are we assuming that the Ca and K are exchanged 100 percent at the water softener? If so that doesnt make sense because I just measured my gH and kH and they were 15 and 4 respectively.

Using that rule provided by the WQA we're assuming 100% exchange of Ca and Mg with K. But with 15 GH that is just too high to have gone through water softener, or somehow it isn't working properly. ](*,) My guess is the water you are testing must have bypassed it.

In my old house that was the case. Only the bathrooms had soft water but the outside taps and kitchen sink did not. I would measure 0-1 GH from the bathrooms but get over 20 GH from the kitchen sink.

Also, is there an actual way to calculate my actual Ca ppm and Mg ppm from just knowing your gH and kH? I mean kH is a measure of CaCO3 but then gH is a measure of Ca + Mg right? So is there a way to get your total Ca ppm from knowing your kH levels and then plug that into the gH equation?

I exactly understand your line of thought. If KH did measure CaCO3 then yes it's possible, but unfortuately KH does not. KH primarly measures HCO3-, but water reports express this as alkalinity in ppm CaCO3. I know its quite confusing...

Since GH measures both Ca and Mg, you need a test kit for one of the elements to find the other. There are several threads covering this topic in circulation.

Hope I didn't cause more confusion, lol.