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.