I put a second wool sock on somewhere in the mid 40's(F) and add booties below 40(F) -- but booties have to be really loose ones to work well with a sandal (like showers pass rain booties with a hole cut in the sole for the cleat). I add a toe warmer chemical pack between the sock layers once we drop below 30(F). I switch to boots once we drop below 20(F). I may adjust all those settings 5 degrees up or down depending on sun exposure and wind. My typical commute is an hour, so you could do with less if duration was shorter.