Combining solutions with multiple concentrations - Mixing NPK fertilizer ratios
Let consider ratio of N/P.
Let fertilizer A has this ratio N/P=a.
Let fertilizer B has this ratio N/P=b.
Let you want the ratio c, where a &lt; c &lt; b.
Then you need to mix A : B in ratio $$\frac{ b - c}{c-a}$$
In the Central Europe with German scientific influence, it is called the &quot;mixing cross rule&quot;.

Note that you can chose just ratio of 2 from 3 elements and only in the range between these 2 source fertilizers. The ratio of the 3rd element is determined by your prior choice for the other two elements.
You would need 3 fertilizers to mix to independently choose ratio of all 3 elements, again within the range of these sources.

A graphical chart method(*) may be more practical than a numerical method.
One can draw a equal-side triangular ternary chart. Each each corner of the triangular ternary plot represents formally a fertilizer with the solely N ( or P or K ) component.
If we take e.g. the N corner, than lines  parallel to the side opposite to N corner represent N relative content to the sum N + P + K. The same for other 2.
Within this triangle, one can draw 3 points, each representing 3 different particular fertilizers.
Then one can draw similar chart between these 3 points for these fertilizers.
Available ratios than fall into this smaller triangle.
Then one can on the big chart choose N:P:K ratios and than read from the small triangle chart the ratios of all 3 fertilizers.

There is small catch in all this. The amounts of fertilizers to be mixed should be normalized for their total NPK content.
So mixing fertilizers A, B, C would not affect the total NPK amount per volume/mass, only the component ratio.

(*) It is also called a ternary plot on Wikipedia. See also the listed referenced there:

an online tool to create ternary plots)
simple Excel template
Trip-Plot Excel package

Or you can used PDF chart template to be printed, provided by @MaxW.

Alternatively, I have prepared on Google docs the spreadsheet,

using linear algebra, which provides the mixing rations for given 3 fertilizer compositions, checking forbidden combinations.