Paul from SWISCO responded:
October 16, 2017
Since your tubes are 12-3/8" long, I would play it safe and go up to 13". Specifically, I recommend our S600-13. The reason you want to round up is to make sure the spiral reaches the bottom of the sill and doesn't come detached from the tube. You can afford a little extra length, but you can't get away with a little less.
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