So I ran the SK10 calculations for WTI Crude Oil - SK10 = [Vol(ATM) – Vol(ATM – 10%))/sqrt(DTE). I have a real-time data grid that shows me the Normalized 25D RR [IV(25delta) - IV(75delta)] / IV(50delta) live for every automated curve fitting publication. Although I think the SK10 helps characterize the downside skew across all expirations of the terms structure, it only tells half the story. Because the SK10 measure only includes the downside strike (which may be appropriate for stocks or equity indexes), if there is a flattening or steepening slope in the calls (like we see in commodities or FX) then one measure may conflict with the other, which it does in this case. According to the SK10 calculation, My front month expiry Mar19 has the most negative SK10 figure (-.206) vs the further out months, implying that its put skew is the steepest vs the ATM, which it is. However, my Normalized 25d RR reading of (-7.37) shows that Mar19 has the flattest overall curve when comparing the OTM 25d puts vs the OTM 25d calls. This is due to the OTM calls (upside strikes) trading much higher vs the ATMs in Mar19 relative to the other further out expirations. It's clear that the Normalized 25d RR indicates a more accurate picture of the overall slope and shape of vol curve across all months under one product. The SK10 measure only gives me relevant information for half of the vol curve. Is there an SK10 formula that includes upside strikes? What would be the mathematical formula for the correct moneyness for an upside strike (OTM call) for a log-normally distributed underlying. What would be the SK10 equivalent for an upside strike? It would have to be some strike more than (>10%) 10% OTM, or less than 90% moneyness.