This graph from Richard Nisley's thread on garment warmth may be what you are looking for.
The entire thread is worth reading.
Also keep in mind that there are other reasons that you may want to choose a Thermawrap over a down jacket. I have a Montbell Themawrap and a UL Down jacket and use them very differently. The down jacket is definitely warmer for sitting around camp but I wouldn't ever wear it while hiking or in wet brush or rain. The Thermawrap isn't as warm but I can wear it without worrying about sweat, wet brush and light showers.