Bill Stock wrote:

I gather these "dissolvability" numbers are cumulative and not specific to
each chemical? Is the cumulative effect straight line or does it depend on
the chemicals being dissolved.

In chemistry the amount of a salt that can dissolve is sometimes
expressed as a solubility product constant. That is, at saturation,
multiplying the concentration of the negative ion by the concentration
of the positive ion gives a constant value. (What the value is depends
on which salt you're talking about.)

To a first approximation, the value of the constant does not depend on
what other salts are dissolved in solution. For example, the amount of
K+ in a saturated KNO3 + NaCl solution is the same as the amount of K+
in a saturated KNO3 solution without the sodium chloride.

However, if the two salts share the same ion, you get what's called the
common ion effect - you increase the total amount of the shared ion,
which reduces the amount of the other ions which can be dissolved,
relative to dissolving each one individually. I've seen it happen with
concentrated HCl added to concentrated NaCl - the Cl concentration got
too high, and NaCl crashed out of solution.