Assume a spherical turkey...

So, you've got a recipe for Turkey with Molasses Butter. It gives ingredients for a mixture that's to be inserted under the skin of a ten pound bird. We've got a twenty pound bird. How best to scale up the ingredients for something proportional to the surface area of the bird when all we know is that we've doubled something proprtional to the volume of the bird? Well, if you assume a spherical turkey, then doubling the volume is generally slightly under doubling the surface area, so that should be OK. See attached graph.

And that's what I did tonight after the mashed potatoes.