Kiwi - when I did mine, I cut them off short enough that there is about a 1cm gap between the halves when on the blades, and then aimed to set the spring screws so the spring would be just tensioned when the guard is off the skate. The idea was that the guard wouldn't flop into 2 halves when not on the blade (which I thought would damage the springs over time).
From memory, the screw holes fell halfway between where I wanted the springs to be - isn't that always the way?? - so I set them on the tighter side, which made it somewhat more difficult to attach the springs. The first screw of each spring was OK (no tension on it), but the second screw was murder because the guard wanted to twist to release the tension on the spring! In hindsight, I might have been better to have cut the guard a smidge shorter then attached the screws on the "looser" side of correct, but I'm not sure.