So, just to confirm, the ratiometric equation is this:
R2 = (R1 * X) / (1024 - X)
Where R1 = known resistor (10 ohms) and X = the value from the ADC (0 - 1023).
(I use 1024 in the equation instead of 1023 according to Arduino data conversion - 1023 or 1024?)
This appears to match @JohnRob's equation in #30 and @er_name_not_found in #38 (among others I am forgetting to mention).