Brs 2 part equal dosing?

[Randy Holmes-Farley]

I tested my tank water and had the lfs test it and the results confirmed all my testing was consistent with theirs. I didn't have them test my mix water. The salinity was 35ppt which I checked with a calibrated refractometer. My tank measures this and my made up water. Lfs also confirmed this with the tank water i brought in. I mix approximately 30 gallons using rodi di water and tropic marin classic.
My testing consistently shows my mag level dropping then I dose an appropriate amount to bring it up to 1350-1400 which it does. It will stay there for awhile before dropping again. My magnesium is going somewhere. Attached are testing results

If the salt mix is actually lower than your target, then of course that will create additional demand when you use it for water changes. If it claims 1300 and you want 1450, then one suggestion is to just always add magnesium into the salt mix. I did that for years with IO which used to be low. I never measured it, just added 150 ppm of magnesium to it.

Overall, I still think you'd be better off not measuring and just adding the recommended amount based on the recipe. But if you really believe your magnesium testing is consistently accurate, then there's no reason to not do that, or at least use the testing to confirm that the recipe plan is working.