The best way to answer this question if you are operating in accordance with JAR OPS is to direct you to JAR OPS itself.
With the best will in the world the answer to this question can vary hugely depending on the length of the flight, whether your company has an established fuel usage monitoring system, and many other factors. You can see this for yourself if you follow the link below.
It's important to bear in mind (as will see from the document) that the requirements are different if you are planning the flight on the ground, planning the flight in the air, or conducting in-flight fuel monitoring/management after the flight has commenced. This may be why the answers to your post have varied a little.
I know the document may seem a bit daunting but at the end of the day it is the Bible by which JAR Ops operators establish their own policies. These are the policies that we pilots must operate to. Operators can choose to require their pilots to carry more fuel than JAR OPs requires but never less.
http://www.jaat.eu/publications/jars/jar-ops-1.pdf