You want to make sure the beam from the radar has a clear shot at the ocean in front of you when you are up on plane, rather than hitting the front edge of the hardtop. This photo may help explain things:

If your rocket launchers get it up high enough for this, it is fine mounting it at the back. You may need to fine tune the angle of the dome once you get it installed... you want it to sit fairly level with the boat on plane (which typically elevates the nose quite a bit vs. off plane). Some stacked plastic spacers on the rear mounting bolts work fine for this, or you can cut some custom starboard shims, or you can buy pre-made solutions online.
You do want to make sure the dome is up high enough that it is above head height of anyone walking around on the back deck when you are operating it. Microwaves do strange things to human eyeballs.