The various ΔX values are based on the value of ΔD. There's a table with the values. Say you have a ΔD of 0.86, the ΔX value is then 0.268. The fractional gradient point is 0.268 log-H units to the left of the fixed density of 0.10 over Fb+f. Take the log-H at that point and plug it into the equation 0.4 / Hfg.
Originally Posted by AndreasT