I've updated my PDF on ink mixing to add what seems to be a very easy way to mix using only simple volume measures. It eliminated the need for any scales in the process.
See http://www.paulroark.com/BW-Info/Ink-Mixing.pdf at page 5 for the formula and procedures.
I intend to continue to use and simplify the carbon printing alternatives.
Paul
www.PaulRoark.com