Sources

0.0408 x diameter² gal/ft

Geometry of a cylinder: a borehole of diameter d inches holds 0.0408 x d squared gallons per vertical foot of water.

1.5 gal/ft

Expert working figure for 6-inch wells. The raw value is 1.47 gal per ft; rounding up covers pore space in the surrounding formation, so the dose is never light.

5 gal

Added volume for the lateral line, household plumbing, and the hose used to recirculate water during treatment.

1 sachet per 150 gal (20 ppm)

One 5-gram Selectrocide sachet delivers 1,250 ppm-gallons of chlorine dioxide (5 g x 250 ppm-gal per gram from SMT's dosing calculator), which is 62.5 gallons at the 20 ppm standard shock dose. Whole sachets, rounded up, and the procedure still ends with a residual test and a repeat if no residual remains.

20 ppm ClO2 (SMT shock range 20-50 ppm)

SMT publishes 20-50 ppm chlorine dioxide as the shock range for initial system cleaning. The kit's standard dose is 20 ppm, the bottom of that range; a fouled well (slime, odor, or a positive bacteria test) is dosed at 30 ppm and the label maximum is 50 ppm.

20 ppm ClO2 vs 50-52 ppm free chlorine

Rough chemical-equivalency comparison for context only: a chlorine dioxide molecule accepts 5 electrons in oxidation against 2 for hypochlorite, so 20 ppm chlorine dioxide is in the neighborhood of a 50-52 ppm free chlorine residual in oxidizing terms. This is not a performance or efficacy claim.