I think Declan's suggestion below is one of the best i've heard.

Take the point of the defender closest to the goal. Draw a line at that point and then extend it to 30cm wide to allow for the margin of error (perhaps 15cm either side of the mid point?) and then if any scoring part of the striker is beyond that then it suggests there is reasonably clear evidence the player is offside.

Allows for a suitable margin of error and should be easy to plot and show on a screen in the ground.

Posted By: Jim, Jan 8, 10:37:03