The average number of compressible strings in a random set of random strings
The answer is less than $2^{n-k}$ of the sequences of length $n$ have complexity less than $&lt;n-k.$ Due to the uniformity assumption on these sequences we just count.

Consider all short programs of bitlength $&lt;n-k.$ Even if all represented sequences of length $n$ the total number of such programs (starting with the empty program) is $$1+2+2^2+..+2^{n-k-1}&lt;2^{n-k}.$$

To clarify,the number of compressible strings by $k$ bits in a the set of length $n$ bits is $2^{n-k}$, their relative number is $2^{-k}$ and their percentage is $100\times 2^{-k}.$ So for each fixed $k$ and all $n$ you get a constant. For example the percentage of strings of length $n$ which are compressible by $k=3$ is $100\times 2^{-3}=12.5\%,$ those compressible by $10$ bits have percentage $100\times 2^{-10}=0.09765625 \%.$