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I have different lengths for different seasons...(I carry a very light UQ rated for 45* or so)
-a 30" sit pad for summer...above 55* at night (nothing needed under your legs)
-a @ 48" mid length...

I would guess you might need more than a sl 1.1 at 180#...lots of stretch
If your stuck on a single layer, 1.7 would be a better choice and be more durable. Your best bet is to ask Brandon via...