>I think the adjustment is in the eye of the beholder. That is, whoever constructed the first pair had some preference, thinking that those two colors go well together. Now we're transposing this, keeping the proportion between RGB components of the first pair, to another pair. I guess the three ratios are not identical, i.e. the pair1X is not simply a darker version of pair1Y, but has a slight shade into any of the R, G or B directions. And this proportional method gives you the same shading over the Pair2X - with the result you may not like.

This is what I thought. However, I wasn't sure about it.

>Perhaps using a single color as the starting point is what throws us off? Why not use the average RGB of the whole first set (to some decimals, perhaps) for Pair1X? Then apply that to the expected medium tone of the 2nd set, and generate the whole 2nd set that way.

The "average RGB" is something that is not clear to me. Could you elaborate more on this as to know what we are looking for and how to calculate it from the fact that we have a first pair for the starting point and simply one color of the second pair?

I would then assume the formula would be good for the medium tone as well.