you don't have to have your boot in place to set the heel DIN, you have to rotate the heel 90° or so to set the forward pressure before you put a boot in though.
and in my experience, using a flathead screwdriver, but turning it sideways, if that makes sense, gives you the most contact with the DIN adjustment, and generally removes the risk of stripping things or chewing up your bindings too badly. like instead of 'inserting' the screwdriver into the screw like you would normally, you hold it perpendicular to the screw.