You are on the right track, but the sizes of scenery can vary up to a limit, as well as height bitmap is different for each scene.
The info for this is sparse.
I've made a Util from Visual Basic that will do the job with less frustration.
You can download it from http://polywop.com/fms/FMSIndex.html
It will allow you to plot the X,Y, & Z for any object you want to place with a mouse click and it will work for most scenery files.
It also has the necessary support files for VB if you don't have them installed.
Hope you can use it.
Perry