depends
you can do all your farming in one go, and in that case you should calculate the total sp needed for your build using a calculator, and farm that much, than put all your skills up for a 0 gap after that
or if you dont plan on going to a higher level then you could farm only a bit, but not too much, then farm later if you need too.
or if you want to farm later at 42 or something, then you can calculate the sp needed to lvl till you have a 33 lvl mastery.
the skills you should put up can be found in ladyB's
guide
the character builder can be found
here
sorry if i misunderstand you, hope that helps =]