Controlling Orthogonal Camera Frame Size?

Abendwind

I might be missing something terribly obvious here, but I have the following problem In DS 4.10: I want to fit my camera frame size x (with perspective OFF) to any exact length l of an object. I suspect there's some basic geometry involved here, but I don't seem to be sure what it is that the camera settings actually do on the orthogonal camera.

More specifically, say I have a 100x100 metres plane, and I want to fit the camera frame exactly to the edge of the plane, so a 100x100 pixels render will yield exactly 1 pixel per metre in my final image. Using auto-frame always seems to leave some wiggle room at the edges, so I'd have to manually zoom in a fair bit if I wanted to match the frame to my plane - with rather inaccurate results. Is there any geometry trick to achieve the desired result more precisely by tweaking the camera settings?

Best Regards