Certainly. I started by finding the clo value of down. Interestingly, the thermal conductivity of down is expressed in Clos/oz/yrd^2 instead of the standard units of W/m*K (or btu/ft*R for imperial). See this source for information on Clo value:
http://www.adventurepoet.com/adventu...ngs-explained/. 800 fill is 1.68 clo/oz/yd^2.
Since Clo and thermal resistance are analogous, the equations became relatively simple. Keep in mind that thermal resistance and R-value are not the same. The R-value that is quoted by sleeping pad manufacturers is actually the thermal resistance per yd^2 (that way it is independent on if you get the longer pad or shorter one).
With that in mind, the clo-value (clo/yd^2) can be found by simply multiplying the clo/oz/yd^2 found at the link by mass of the down in a volume which has dimensions of 36 in *36 in * height (the loft of the down in the sleeping bag). Then multiply this value by 0.88 to get the equivalent R-value.
The equation broke down to: R-value = density of down (oz/in^3) * loft (in) * 36^2 (in^2) * clo-value/oz (clo/oz/yd^2) * 0.88.
In this case it is: 1/800 (oz/in^3) * 2.5 (in) * 36^2 (in^2) * 1.68 (clo/oz/yd^2) * 0.88 = 5.9875 (R-value)
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