How can I determine the resolution of an image to obtain a specific error rate?

 Hi all! I am verifying my stereoscopic calibration by measuring sides of a triangle formed by colored markers Red,Green and Blue. Given below is the pixel to mm conversion of two resolutions which I experimented with and 1024x768 gives me an error of 1mm when compared with real-world distance whereas 640x 480 gives higher error rate. I need an error of 0.1mm. Is there any way in which I can find the resolution of the cam that would give me 0.1 mm?

*Resolution*           *Value of 1mm*

1024 x 768            X=0.1758 Y=0.1693
640x480                   X=0.2831  Y=0.2708