Check mag, make sure it's 1350 give or take about 50, and bump the alk up with just enough baking soda to take it to about 3-3.5 always make sure on the volume entry in the calculator to only enter water amounts not water and rock. And yes that is a guess. So if you have 280 total system, I'd guess 225 gallons. On my 245 right now, I always use 200 and it tests out exact. It surprises me that the rock takes up so little actual water volume. That's due to all the pores and holes in it.