I don't know if the rules have changed but it used to be that you required enough fuel to get to destination, hold for 30 mins, route to alternate and have enough to hold there for 30 mins, then land.
WetFeet, for despatch you only require 30 mins holding at the alternate. For the sake of accuracy you also have to add contingency (usually 5%) to fuel required to destination and fuel to get to alternate.