This may be an oversimplification, but why don't you weigh a board (with all
the components on it) prior to the wave solder process and then weigh it
again after the wave solder. The difference in wieght will be the solder used
and you can establish a rough solder usage estimate by determining the number
of plated through holes and surface mount pads on the solder side of the
board (plus estimating the relative size of these holes/pads).

This should give you an approximation of the amount of solder you would
expect to use for each board design. You could even keep a log so that more
exact solder usage numbers would be available for repeat jobs.

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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