Your calculations sound right. There's a chart on the Seachem site for ratios of Acid Buffer to Alkaline buffer to achieve a specific pH:
Alkaline Buffer
To be honest, unless you have really sensitive fish, your well water pH is fine. Does your test kit give a reading in ppm or dKH/dGH for your hardness and alkalinity (carbonate hardness)?
With the AP tests, fill to the line. Apparently they're made to be measured with the
top of the meniscus at the fill line, contrary to what many of us may have been taught in science classes. They're not lab-grade tests, but they do give a good enough indication of the levels in your tank to ensure the health of your fish and plants.