Calculate the total capacity required. This is the sum of the tooth differences between your large and small chainrings AND your largest and smallest rear cogs.

So....

Capacity = (52 - 30) + (26 - 12) = 36

Shimano has increased the total capacity in recent years. The 6500 series had capacities of 29T (SS) and 37T (SGS). The 6700 series bumped that up to 33T and 39T, respectively.

A mid-cage would be the correct match for your drivetrain. Some people have spoken of running an SS in your situation, with the requirement of staying out of all but the few largest cogs while in the small ring (to circumvent the short cage's inability to take up the excess slack).