I sympathize, I went thru all this with Terragen/Maya -and eventually wrote my own scripts once I'd worked it all out - but it took a while to figure out.
QuoteI've checked the camera settings as they all seem to be match!?
Did you check the Use Horizontal/vertical Fov on the Terragen camera? As Matt mentioned this can sometimes be set to the wrong value.
QuoteI also had to 'Add Height' on top of the multiplier
You shouldn't have to do that - just experiment to find the right multiplier.
If you think the problem is the wrong multiplier for the terrain generated from the raw file in Terragen - if you have the pro version of Terragen, export the terrain via the Micro exporter.
Bring this into Maya and you should have a terrain that matches your Terragen scene. The height might be different, so you might have to tweak your camera animation to compensate.
Or go entirely Terragen, just bring in your trees and plant them manually or use the populations to put them in the scene. You can use the painted shader to paint a map of where you want the trees to appear.
Here's a link to some tutorials I've made that might help give you some ideas:
https://www.youtube.com/watch?v=bTKfAeEPDhY&t=912shttps://www.youtube.com/watch?v=ALbWYn_YHMI