Size it using the big-big + 1" (or in your case only-big+1") technique. That assures you will be able to use your largest rear cog safely. Here is the technique taken from Sheldon Brown's web site (emphasis is mine):
"The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length.