I have a question regarding what start height to make the trajectories and the correction factor for MAGL…
For my case,

MSLP=1026 mb
PRSS=995 mb

Difference = 1026 – 995 = 31mb
Multiplying 31 times 10 gives 310 m
The actual terrain height at that location is 244 m

244 – 310 = -66

This means that I would start a trajectory at 66 m below the start height that I choose for this site. All of your examples in the frequently asked questions suggest that the start heights be adjusted by adding not subtracting height. Is subtracting 66 m from the start height valid?

The case you described is a valley. The large-scale average terrain is higher than the actual location of interest. There is no correct answer in terms of what you should do. If the valley is relatively wide and large, then I would ignore the terrain issue and start the trajectory at the same height above model terrain as above the actual surface. If the valley is narrow, then you should probably start at ground-level regardless of height.

Roland Draxler