If you are not skilled at working with toxic compounds, I'd recommend against DIY fluoride.
"38ML of his Fluoride solution would raise 0.5 mg/L in 378L"
Thus the dosing solution must be 5 g/L fluoride
Sodium fluoride is 45.2 % fluoride by weight, so the dosing solution can be made by mixing 11.1 grams of anhydrous sodium fluoride in a liter of water.
Be very careful with sodium fluoride as it can be quite hazardous and toxic.
Hazard statement(s)
H301 Toxic if swallowed.
H315 Causes skin irritation.
H319 Causes serious eye irritation.
Precautionary statement(s)
P264 Wash skin thoroughly after handling.
P270 Do not eat, drink or smoke when using this product.
P280 Wear protective gloves/ eye protection/ face protection.
P301 + P310 + P330 IF SWALLOWED: Immediately call a POISON CENTER/ doctor.
Rinse mouth.
P302 + P352 IF ON SKIN: Wash with plenty of soap and water.
P305 + P351 + P338 IF IN EYES: Rinse cautiously with water for several minutes.
Remove contact lenses, if present and easy to do. Continue
rinsing.
P332 + P313 If skin irritation occurs: Get medical advice/ attention.
P337 + P313 If eye irritation persists: Get medical advice/ attention.
P362 Take off contaminated clothing and wash before reuse.
P405 Store locked up.
P501 Dispose of contents/ container to an approved waste disposal
plant.