For temps much below 25f I go with more insulation in the boot, not more sock on the foot. If your boot is good enough you shouldn't need more than a thin liner for a sock. I've routinely gone with nothing more than that in my bunny boots and been toasty. The sock is really just there to wick up any sweat. The warmth comes from the warm air around the foot, and you need boot insulation to keep that intact. The old style felt boots are hard to beat for insulation but very bulky. For this side of -40f. you can find boots with enough thinsulate to keep you warm.