Yeah, dual gauge can make things like this tricky. The NMRA HO gauge shows 4' as the dock height from the rail head. The HOn3 gauge shows 2.75' for that, but I just round to 3' in most cases. When both gauge cars will use the same dock, I just go with the lower height, which would be the case in real life as the NG facilities would generally come first in real life.
It generally helps to have the common rail at dockside, but it's not always possible. Be sure to test clerances with both SG and NG cars.
