Your gearing will require your rear deraileur to "wrap up" 32 teeth ((53-39) + (30-12) = 32) worth of chain. Shimano says the current short cage Ultegra has a capacity for 33 teeth so you are ok. Even if it was a bit under capacity, it could be used by avoiding the small-small gear combination.

However, check to be sure the 30T cog will clear the derailleur's jockey pulley. Shimano claims a maximum rear cog of 27T for their older designs and 28T for the newer ones. Experience indicates that they usually clear 30T and sometimes 32T but you have to be sure it will work on your particular bike.