"Easy" — but impractical to calculate in practice — concrete answer: it's the information-theoretic co-compressibility of the all the dependent information required to construct one individual's proteome relative to another indivdual's.

(I.e., if you have all the DNA + methylations et al of one person's genome, stored in a file, which you then compress in an information-theoretical optimal way [not with a general-purpose compressor, but rather one that takes advantage of the structure of DNA, rearranging things to pack better], and then measure the file-size of the result; and then you create another file which contains all that same [uncompressed] information, plus the information of a second person's DNA + methylations et al; and you optimally compress that file; then by what percentage is the second optimally-compressed file larger than the first?)

Or, to use a fanciful analogy: if we had a machine to synthesize human cells "from the bottom up", and you had all the information required to print one particular human's cells stored somewhere — then how much more information would you need as a "patch" on the first human's data, to describe an arbitrary other particular human, on average?