That’s close enough, it’s not an exact science.
If SH is stored in the dark (i.e. a box) and is kept at a temp below 80F, it has been my experience it looses roughly one percent per month. So if it was packaged at 12.5, which is rarely is (it’s usually more like 14) it’s still likely at an 8 or so.
This website is how I calculate my dilution ratios.
Go to the ppm ppb section. Change ppm to % and ml to gallon. Input 12.5% in the top left box then however many gallons of bleach you have in the top right box. Enter the % you want hitting the surface in the bottom left box and hit calculate. Bottom right will show your total gallons. Subtract your bleach from that and that’s how much water you need.