This will not resolve the problem. It's like putting a band aid on a wound which needs 40 stitches. Where did they get 10cm from? Looks like an arbitrary number to shut everyone up that we are taking margin of error into account so we are now accurate. So in your example saying they are 10cm+ a toe offside can be completely wrong and the actual fact (the reality, not what TV screen shows) could very well be they are 10cm on side because the actual margin of error was -20cm. What the actual margin of error is dependent on many things (see below) and just picking 10cm is as bad as not taking a margin of error into account in the first place.
- Camera speed (frames per second) - this impacts if we are freeze framing at the exact moment of first contact with the ball
- Resolution of the camera - This will impact determining the 'end points' of attacker and defenders
- Bandwidth of image/video transfer - camera to software then to VAR display - same as the two above
- Resolution of VAR viewing screens - same as 2
- Accuracy of line drawing - both vertical and parallel lines as well as parallel-ness to goal line (angle)
- Distance of SLD defender and offending attacker to camera - This impacts resolution as well as accuracy of line drowning. the closer to camera, the more accurate
- The speed of movement of both attacker rand defender - the faster the move the bigger the margin of error
- Angle of camera to location of offence
I may have missed a few here. But you see just saying give a margin of error of 10cm is not good enough.
For me, either fork out some money and get high tech equipment and software with negligible proven margin of error (a few mm) with hawk eye type images for 'factual' decisions, or remove all the technology stuff and make it a subjective decision. If it is not obvious by just looking at replay that the AR has made a mistake, stick with the AR call.