If an ever you find yourself working with percentages always recalculate them as fractions. In fact with solutions you are working with probabilities of encountering a given molecule (loosely speaking).

So with mixing solutions: You have X amounts of P(x) solution, and you want to add any amount Y of P(y) solution so it will in the end be a P(z) solution. Now you are adding weighted probabilities.

I taught my then elementary-school-level sister how to convert percentages to fractions and fractions to percentages and she has never since used specialised algorithms for percentages.