In reply to Little Dave of Sheffield:
My feelings as someone who runs reasonably warm in his core is that this is right.
With fair layering, I don't think I'd be reaching for my warm until it was below/around about -10. By this I mean I'd prefer to swap for the warm than swap a high pile fleece for my powerstretch, and I don't think I'd need both until way in the minus.
This is of course assuming you have appropriate insulation for when you stop, but that's not the baselayers job.