Ok...
my calculator is this.
The tank is a certain litreage. This will never change unless I upgrade or downgrade.
The litreage is consistent.
By testing,adding and retesting has set the amounts required for my litreage.
Ie: I know now that for my litreage I need 1 heaped tablespoon to raise MG by 10ppm.
So....If my tank reads 1290ppm, (As it did this morning), I simply add 6 tablespoons to raise to 1350ppm.
Same with KH and Ca.
As this is a known, I dont need a calculator. It is already calculated.
I just feel it is more trustworthy.
If it works for you guys, then thats great!