To find the minimum chain length, loop the chain around the big chainring and 32T cog -- bypassing the rear derailleur -- and add 1". The whole system may be happier with a slightly longer chain, so if you have a reusable master link, you might start out with a chain that is a couple links longer than the above formula and see how the rear derailleur responds when you're in the small-small combination. (If the chain is too long, the lower derailleur pulley will swing up all the way, and parts will start bouncing and rubbing on each other -- not good.)