I dont know about the magic number, but you can use multiple cameras and leave the scale alone. just make your HUD camera a child of your main camera. use the culling masks of your hud camera to only show stuff on the HUD layer and the main camera to show everything but the hud. also make sure that the depth of your hud camera is bigger than the main cameras. hope that helps

I had no idea multiple cameras could be used to create a single view. This sounds like a good solution but I've may have run into an issue.

I've added a 2nd camera and made it a child of the main one. I created a layer called HUD and the 2nd camera only has the HUD layer checked, while the 1st has everything but HUD checked. The main camera's depth is -2 and the HUD camera's depth is -1. When I go into my game, I can only see my HUD. What's odd is I can see objects in the main camera's preview while in the edit mode.

I've also tried not having any culling on the main camera and tell it show show everything. But I still only see the HUD layer.

edit: Ok, I found that I needed to set the Clear Flags in the HUD camera to depth only. This works perfect! Thanks!

Set the HUD camera's "clear flags" to "Don't Clear".

Note that using two cameras is more expensive than using ony one camera.

I'm using the camera in my game like this. The relative scales are:

1 - sphere, cube
10 - plane
0.5 - camera (sphere/cube will fill screen at 0.5 orthographic size)