If you want a really fine slot you won't get any finer than a jewellers saw, blade thickness starting at .15mm up to .42mm. As you can see these are very fine blades to work with. Again depending on just how fine you need to be, what we over this side of the pond call a junior hacksaw will give you a slot width of just under 1mm, around about .75mm, measured by eye and magnifying glass. Remember if you are not over practiced with hand tools the initial start of the cut, due any wobble which may occur will mean that the opening of the slot may be slightly wider or as we say bell mouthed. This can be remedied by allowing a little extra depth of screw head and then filing down to finished size. Hope this helps.