Your calculation is actually this:
1 ml ÷ 3785.41 ml (1 US gal) x 100 = 0.0264%
0.0264% x 10,000 = 264 ppm
10x more than you think.
Also, when you say "elemental", SiO₂ is actually a compound of which Si makes up 46.75% of SiO₂.
The instructions on the label state 1 tsp per gal = 1 ml per...