Just a suggestion for TRADES only - avoid the mathematics; just use the actual face values. Trader 1 sends $6.45 of face value (22 plates) to Trader 2 in exchange for the same $6.45 of face value (31 plates).
There are only a few (post-1950) plate blocks where the catalogue value is far in excess of the face value, and those could be handled on an individual basis. The perfect example would be the 15¢ perf 12 John Paul Jones #1789B. A plate block of 10 has a face value of $1.50 but a catalogue value of $37,500.
This method won't work as well for plates before 1950, and is useless for those before 1930.