Your calculations figure pure/distilled/RO water at perfect density? (this is 1g/mL H2O.). If so, this totally works and is accurate enough for our purposes, but it is important to understand that really you are making an approximation that you have pure water, when in reality you probably don't (we have fish and substrate and plants (organics and inorganics), we add fertilizers, and we may/may not have leeching ornaments (driftwood/rocks) and/or tap water). So, another way to do your calculation is to keep it simple:
ppm K is really mg K/L tank
How much K is on KNO3
How much K is in K2SO4
[(mg KNO3 * mg K / mg KNO3) + (mg K2SO4 * mg K/mg K2SO4)] / tank volume in L = K mg/L in your tank = ppm K in your tank
But again, ballpark is totally good enough. You just want an idea if what you're adding to the tank; your plants just care they get all the nutrients they need to grow. Nobody (plants, people, or fish) should care about exact values! How far off has your calculation been?
Cool site btw! I had not seen it before.