I really wanted to find out exactly how much etchant would I need for a given board. That would be at least a theoretical minimum to etch all exposed copper, so after some calculation I made this table. In practice, you would probably want to double that amount to get it to etch faster, but that's a good starting value to measure.
I have ferric chloride, what can i use from this calculation?
If you find the "Efficiency" factor (how much copper can it etch for what amount of its own mass or volume) then you can ignore B7:B8 and the first half of the table still calculates the total amount of copper in B10:B14 for a given board parameters (plug in values to B2 through B6).