Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
All the systems are flexible enough that the variations you are asking about are small.
I also use spoons to measure the ferts.
Tank volume is variable, but I just call each tank by its commercial name (45, 29, 10... ) and dose accordingly, unless it is a paludarium, or has a very large sump that makes a difference. A tank such as a Rift Lake tank with a solid rock wall might really be 20% less water, but most of the 'scaping I have seen here is pretty typical: Couple or so inches of substrate, maybe a mound, a few rocks, or a branch or two, and a canister filter. Not enough to seriously affect water volume.
There will be more variables in the lighting and CO2, and you will be adjusting the dosing as you go, anyway.