The 146 contaminated runway performance works the same way as Alex has outlined (if i remember correctly) .One could calculate a Vstop and a Vgo , you then reduced your take-off weight as much as poss' to try and get the two close together .
There was a caveat in the book that stated the data was advisory in nature and had not been validated by flight test !
Try explaining that at the Board of Enquiry >
With regard to the generic 10 knot reduction , again the 146 was similar , there was a table in the performance book which gave you a number i think between 7 and 10 knots depending on runway length which gave you a Wet V1 (usual Vmcg caveat applied).