737incognito
No one can give you an answer as to exactly how much extra as it is extra over and above the legal requirement and comes down to the crews judgement. Personaly I will add either 15 or 30 mins. 15 is another approach and 30 conforms to company guidance on LVPs. I have carried more, every case is individual. As has been mentioned the key is to have thought it out and not to carry a random figure just for grandma. I have also refused to dispatch as Dest was below planning minima and the only 2 legal divs were very marginal and we were carrying a snag that meant we were Cat1 only. Our company provides us with 4 divs on the plan by the way. You can always ask ops for new divs if you don't like the ones you have, they are very helpfull mostly.
At the end of the day the rules have to be observed but within those rules you have to exercise airmanship. Diverting in relatively poor weather worries me less than diverting in good weather. In poor weather I will make sure I have an appropriate fuel load and options which are monitored during the flight. If I don't like it I don't continue. In good weather chances are I'll be carrying plan ie min legal. If I divert now chances are I'll be close to min fuel along with loads of other guys in a similar fix. Could be quite a bun fight when the first pan\mayday goes out and it all ripples down the line. Anyway I digress.