I try to get the gap as small as possible where the tubs still pull out easily. For BP hatchlings 1/8" is okay if the tub and rack material are really sturdy, but if the tub plastic or rack is flexible, they might be able to put enough pressure to make a bigger gap and squeeze out. For smaller hatchlings, even 1/8" is too much, so if you are considering ever getting any other species, you might want to make it less.
What kind of materials and tubs are you using?