I have a 20-30 gallon sump.

I also have a 3" layer of substrate.

I figured that the sump volume replaces the water volume displaced by the substrate, rocks, wood, etc.

So, I figured 180 gal volume for dosing..........

Maybe your setup would work out the same, 66gal sump vs displacement of subs/decorations so maybe it is moot point?

I am also interested in Tom's opinion on this........

What type of substrate ARE you using? If nutrient rich, perhaps your EI or trace dosing can be slightly less?

I agree that you should adjust as you go along until you find your optimal strategy........

Vaughnh, your opinion on this?