Starting around 40F I'll put on a light cap. Best I've found is the Minus 33 Reversible Shade Beanie, as it's nice and light and it comes down over my ear lobes. Under 20F or so I'll switch to a light balaclava, then around 5F I'll go with a midweight balaclava.

Hands and feet are the most challenging. Around 35 degrees I'll but light wool socks over VBL socks in uninsulated shoes (fat, mtb and commuting I wear Five Ten Free Riders -- this being Minnesota, I tend to retire the road bike November through March) and then I will switch to insulated boots around 20F and user lighter or heavier socks as needed. From 50F to 35F I wear light fleece gloves, switching to midweight windproof at that point down to about 15F-20F. Around 5F the mittens or pogies come out.

Upper body is pretty simple, with light and/or midweight merino under windproof top. Legs are even simpler, with lighter windproof pants 50F to 25F and then the heavier pants down to 5F and long underwear below that.