AS all the guys have said for Dispatch one needs the 1.67(dry)or if the wx at dest is below 300'/wet one needs the 1.92.
Nowadays once airbourne,should a systems failure develop check the ACTUAL landing distance in the QRH/AFM/FCOM ,and factor this distance as advised by the increment required for the failure(loss of braking systems/hydraulics.etc require 2.1 total distance)..if not always ask for the longest runway or divert to a good into wind runway..
