OK that makes sense, but so did your previous math...after a while (ahem

). Dilution math can get tricky because it's not exact & then it gets more iffy over time.
This is why a TDS meter helps in the long & even short term. There can be dissolved solids that are slowly increasing (from ferts or rocks, whatever)...& try as we may we don't ever feed, WC or apply ferts the
exact same amount. & then there's seasonal water issues. I'm in CA too, I know!
Just spend $13 next time you order from Amazon to get a TDS meter. I guarantee that it will be easier & more exact to your tank than trying to figure water dilution by math possibilities alone...I miss my TDS (It's almost a song...I miss my MTV...I think you know

)
Do the best you can with WCs but if you really want to know you need a TDS meter...even a cheap 1 helps.