If your chain is the "right" length now, then you shouldn't need to bother. I mean you already use a 21 tooth (or there abouts) now without problems, it just so happens that this will is where gears finish with the new cluster.

The biggest issue with chain length, IMHO, and which I trashed some gear 'discovering', is making sure the chain is short enough on the 12 tooth when in the big chain ring....the Shimano rear deraileur pamphlet that comes with the deraileur says the main wheel axle and both jockey wheel axles should form a straight line perpendicular to the ground. That's a reasonable amount of tension, and I think a lot of people don't have that much. Shifts are sharper and chains are thrown less frequently when the chain is that little shorter and under the proper tension by the deraileur.

IMHO, checking this tension in your hardest gear (12*53) is more important than worrying about how 'extended' or stretched the deraileur looks in your smaller gears when the chain is a little shorter.