Replying to New boots binding adjustment

I got full tilts in same size as my mojo boots. 26.5. but the sole length on full tilts is 301mm and on mojos is 307mm. so the boot is obviously lose a bit. I have griffon bindings so I know I can use a screw driver to adjust the length but how do I know when to stop? is there something i should be looking for? thanks I couldn't find much on that by search. PS. I wanna use them tomorrow so no time to go to shop by then.