I like my pad nice and squishy, with just enough air to keep my hipbone off the ground when I'm on my side. My goal is to have my spine parallel to the gound when lying on my side. IMHO, lying on a fully inflated pad is not much different from lying on bare concrete.

Underinflation does reduce the R value of the pad, which is one reason I need a warmer pad.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey