There is no specific measurement for the distance between the top of a Bartolini pickup and the bottom of the strings. The reason being: you cannot get too close, at least not as far as the magnets and coils are concerned. You, however, can get so close that the strings will hit the pickup when you play and this distance changes with styles and individuals. Start with the outside strings about 4mm or 3/16th of an inch away from the neck pickup. If the strings stay clear of the pickups as you play then raise the pickups until the strings start to strike the pickup as you play, play hard. Then back the pickup off bit by bit until all is clear.  Adjust the bridge pickup for even volume output. If you can’t get it loud enough before the strings start to hit it back off the neck pickup again a bit.