The performance charts in the aircraft flight manual should be all that one needs to calculate the weight restrictions/limits when working hot and high.
When departing from a lower altitude take off point to a higher altitude landing point, using the standard lapse rate for approximate temps at the landing site can be used to determine the max landing weight up there.
Working out expected fuel burn for the trip, one can determine with some degree of accuracy the max departure weight for the mission.
Combine that with the standard mountain/high altitude operating procedures and you have a workable plan. Well, Plan "A" anyway. Always have a Plan "B" and Plan "C" and so on for unexpected encounters along the way.