You say you have the P20 already. Try putting it on to see how close it comes to the balance point. If it doesn't quite get to where it needs to be then the extra length of the P30 just might do the trick. If the P30 still isn't long enough to do the job then it is time to consider returning it and getting a longer one.
I must confess to being in the school of "Let's try it and see what happens!"