Rounding to the value that's nearest in linear light is not exactly right (if anything evaluating distances in linear light is a bad idea in general, but that a

Rounding to the value that's nearest in linear light is not exactly right (if anything evaluating distances in linear light is a bad idea in general, but that aside). What you want is to find out exactly how much of each colour to blend in order to get closest to the actual value and blend the colours in those proportions. The formula in the OP is the right way to do this.