Ever watched a paint supplier do a custom tint for a factory color? It's definitely done by weight, at least at every shop I've been associated with. Put the mixing cup on the scale, and run the specified amount of base color or tint in, then move on to the next color. If 1/4 pint of a specific color is needed for a spot-paint job with a trim gun or an airbrush, no problem, and no waste. Those tinting colors are E$$PENSIVE!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!