Converting solvePnP tvec

edit

I am using solvePnP to figure the rotation and translation of a fiducial marker relative to my camera (a Kinect). I understand that the units it will return for translation are the same as those I pass in, which in my case I think is the camera intrinsics:

solvePnP(Mat(markerPoints), Mat(sceneCorners), intrinsics, distortion,rvec, tvec, false);

How can I convert the translation result to real world distance? Would I have to basically apply the fundamental camera matrix or is there a simple scaling factor I can apply? My current thought is to multiply tvec by the size of the marker (in my case an 8cmx8cm, so multiply by 8).

EDIT:

Having gained a bit more understanding, here is what markerPoints are:

//FID_SIZE = 8.0cm
markerPoints.push_back( Point3f( 0.0, 0.0, 0.0 ) );
markerPoints.push_back( Point3f( FID_SIZE, 0.0, 0.0 ) );
markerPoints.push_back( Point3f( FID_SIZE, FID_SIZE, 0.0 ) );
markerPoints.push_back( Point3f( 0.0, FID_SIZE, 0.0 ) );

And sceneCorners are in pixels. Still can't quite get it to make 100% sense.