In article j3Svg.78949$ZW3.42799@dukeread04,
"Jim Macklin" wrote:

15 is the bottom of the green, I like 17 because going below
that can shock cool the cylinders. 12-13 is too low because
some times you might be committing to a landing, [soft
field] with more power than that. Just my preference.

IIRC, the Cutlass/C172RG is set to go off at 15-inches MP.