Sorry, for some reason I cannot wrap my head around this process. Maybe I can break it down in how I think your saying it:
1) my goal is to dose 3.0 mg (per dose or total?) based on my 50gal system volume and your equation above.
2) I take my 11.5mg Interceptor Pill (my dog is medium so doesn’t have the large pills) and crush it up into powder
3) since you took a quarter of a pill (~ 6mg) and mixed it in 100mL of water, I could take 1/2 a pill (in my case, ~6mg because my pills total are only 11.5 vs yours were 23mg) and mix it with 100mL of water. This should give me the same concentration.
4) then I take 20mL of this solution and dose it to the tank. The remaining 80mL of the solution has to be discarded because it’s been in water and will degrade.
5) I repeat the above steps 3x a week for 3 weeks
Did I understand that correctly?