Good news all around. So glad you got the heating set up and that the breeder took the heat lamp back. The temp is measured on the glass under the substrate. They will burrow down if the surface is to cool, and I can confirm that from experience.
A mamba, eh? I like my mambas behind a thick piece of glass!
Slinky seems to be on the larger side, but there is no clear correlation between age and size so Slinky is Slinky sized.