Check your local building code for the requirements in your area. Height, spacing, member size, # fasteners, etc are usually dictated by code. When you, as an engineer, are designing a structural ridge, you will be able assign a height in your analysis. I factor in attic space requirements (head room, ceiling joists, rafter spacing, etc) and then use a 2D Plane Frame Analysis to size the collar tie and connections.
Typical collar ties are in the upper third of the apex.
The International Building Code has a significant amount of changes/improvements in this area from the SBC....I will try to find the section on collar ties and post it back here.
