Best endurance is min drag and I would like to know a way of determining what that is while I am loading up the FMC on the ground.
As per the FCTM, as well as the hold page, to all practical purposes the standard clean speed derivation of Vref 40 + 70 (UP speed) would be min drag or close enough. +10-20 kts probably would make no significant difference due to, as I understand these things, a very flat drag curve. Obviously any slower and flap is required so not applicable.
As the FCTM states, another consideration, is that at altitude, UP speed is less than the min manoeuver margin, so the speed would have to bumped up a bit, Vref 40 + 100 is recommended.
I suppose this can be checked if one actually ends up in the hold, select UP speed and note the engine params, then the same for best hold speed. See if there is any measurable difference.