visible world limits with different screen resolutions

[roguetreasure]

I am writing a game for android phones using the irrlicht-android port.
The game uses a fixed camera view.
When using a phone with 480x854 resolution I get the perspective and visible world I want by placing the camera at (-180, 200, 0) and looking at target (0,0,0).
The problem is that when running on a phone of a different resolution (ie 320x480)the amount of the world visible is vastly different. I need a way to have about the same amount of the world visible no matter what the screen resolution. I have tried using the ratio of screen resolution like
(-180.0f * f32RatioY, 200.0f * f32RatioX, 0.0f) but no avail.
What is the best way to keep approximately the same amount of the world visible at any screen resolution?