As long as you, or let's say we, are flying in Europe with lots of aerodormes available to legally land, we have lot's of options. It's different if you do long haule or fly according to JAR in areas where the options are limited.
It's true and I fully agree JAR gives us, as well as our companies, a lot of flexibilty in planning and operating.
In regards to your concern "... as Captain you can take extra... but how much excatly?" I have to say that all airlines I flew for in my career offered an OFP where at least three different destination alternate aerodromes were stated including the required ammount of alternate fule. It maybe that your present company doesn't offer this kind of service, a service it should offer to you, I think!
However if you don't have this data available I am sure you have access to the performance manual of your current aircraft. There you should get some figures to make up an idea of required fuel figures. I know it's an answer that does sound very arrogant, but may be it's worth a try.
Finaly keep in mind that you can always do re- planning in flight and more important to me, you can land at any aerodrome which is suitable, you don't have to fly to the destination alternate aerodrome stated on the operational flight plan or on the ATC flight plan!
I think that the chance to see a change in the rueles regarding fuel planning is not realistic, so we have to live with those rules.