I must be warm blooded because I've been out in a single burton baselayer and goretex shell in -15c. I'd probably aim to wear an extra merino base layer and maybe a tshirt at that temp ideally. Below that, i'd probably get a hoodie on too and zip the lining into my 686 Smarty pants. Riding generates a lot of heat!
We are all atheists about most of the gods that societies have ever believed in. Some of us just go one god further.