A fluid/liquid ounce (measured in a cup or with a measuring stick) is not necessarily the same as a weight ounce (avoirdupois ounce).
If the components to be mixed have similar densities, then mixing them by weight/scale or mixing them by volume would result in the same ratio.
My PPG sheets show mixing by parts/volume (not weight).
Most likely this does not really matter because the ratios are probably not very strict/critical relative the error of mixing parts using a scale vs using volume.
For small mixes, I bet that using a scale would be far better than trying use volume (no matter what the densities of the components). The errors in trying to measure small volumes are surely more than the density errors from using a scale.
