Your math sounds good, but it isn't necessarily true.

When you are mixing, volumes can shift. You can add a quart of hardener to a galloon of paint and end up with less than a gallon and a quarter of paint. When liquids are combined, you do not necessarily get the additive volume. For paint, it should be close in most cases.

Weight can be more accurate, though you must account for different densities of material. For example, TiO2, the pigment in white paint, is very heavy. Blue pigments are a lot lighter, and there is much less pigment in a gallon. If you use that methodology, make sure you account for the different densities of paint.

Frankly, mixing via the recommended separate volumes will work fine.

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