Optimum fractional vaporization for most thermosiphons is in the range of 20-50% for organic systems, however thermosiphon operation for vacuum systems generally tends toward the higher end of the range.  The boiling point elevation due to static head requires that the driving head be low and therefore so is the circulation rate.  The system must be analyzed including all inlet and exit piping and nozzles and boiling curves at several different pressures need to be used in order to properly account for the static head effect on the boiling fluid.  The use of a program such as Xchanger Suite by Heat Transfer Research, Inc. would be the best way to analyze such a system and determine the minimum delta T for this case.