So everytime I have a brake action less than medium , I consider it contaminated.
Not allways true. Contaminated and reduced braking action are two different things, you can have reduced braking action without any reportable contamination, and not so bad braking action with heavy contamination. In fact acceleration is the main take off consideration on contaminated runways vs. braking on reduced braking action. That is why you have to check both performance charts for contaminated winter ops and take the more conservative one (well, at least we have to).
Apart from that, we do have a boeing landing distance table that gives landing distance for dry, wet, braking coefficients and reported braking action. All factored of course (dispatch info) with corrections for wind and slope, those have to be used inflight too.