Enough!
The trick is always to be comfortable with your decisions. I fully accept that simply adding an arbitary amount "for the wife & kids" on each and every sector is commercial madness, but when the situation requires extra then, by all means, take it.
As an example: Winter weather blankets Western Europe. Fog and snow almost everywhere. Flight planned fuel from MAN to AMS was 7000kg using Boeing 737-200 equipment. I actually took over 15 tonnes as my evaluation was that it was unlikely we would get in either at AMS or the alternate DUS. Ops were insistant that as we were CAT 3 we should depart on schedule so off we went. A few hours later back at MAN we landed with under 3000kg left having been all around the Netherlands and Germany as visibilities dropped to 50m. The only two major airports were left MAN & PWK.
Throught all this I was able to take my time making decisions as fuel was not a problem and things were working out much as I presumed. An extreme example I know but as they say "I learned about flying that day"