Strictly speaking (I was a coatings chemist), knowing a ratio of white to black does not necessarily give you the correct color unless you also know how much white pigment and how much black pigment are in each base color. An additional complication is knowing exactly what pigments were used. At that time, lead oxide and zinc oxide were most often used in white. Todays paints typically use titanium dioxide which is much "whiter" than lead or zinc oxides. For black, the pigment used then was often carbon black which is also true today. However, todays carbon black is much purer and hence blacker. Older carbon blacks tended more toward a brownish black.

I don't wish to complicate things for you, just throwing this out as a bit of knowledge. The best way to match is with a colorimeter although, in our cases, the Mark I Eyball is often good enough.

Dean