In the case of contaminated runways, impingement drag, retardation affects, lateral control and the benefits of ploughing are taken into account. As for an average figure, there isn't really such a thing with today's computerised programs, however with our old Classics we used 10% of the runway limit weight as an average reduction.
However I do stress that under our regulations, contaminated runway data isn't certified so we legally do not have to account for anything, but if we do make corrections, we can calculate takeoff weight without accounting for an engine failure
But there is a difference between legal responsibilities and moral responsibilities.
Mutt