I would say upper 50s for a single layer top and bottom, no seams ripped. Kicked off the TQ last weekend when in the mid 60s - too hot.
Tossing in the 3rd quilt on the bottom gets me into the upper 40s (no seams ripped or modifications - just stock with channels sewn down the sides for the UQ).
Note that I usually switch to different gear when temps are expected below 50 so I've only done the 40s once with just the Costco quilts (unexpected cold snap). I had to be very careful to avoid "cold spots" but once adjusted I was warm enough.
BTW, these are back at my local Costco.
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