Lasercut 5.3 has a bit of a slack way of dealing with the z control, you nudge it up and down a bit until it's in the right place according to the height tool. The amount depends on how long you hold the button for. One of our customers wanted to be a bit more specific about how much the Z axis moves, that way they could make 5 passes on their material and drop the Z axis by 1mm each time. So I made them a custom Z axis controller, this one connects to the PC and allows you to send specific mm commands to the Laser cutter. The PC end is a simple interface that allows you to send 0.1, 1 and 10mm commands to the controller, but it also allows you to send a custom distance. (software bundle here)
The PC controller actually detects when the USB serial port of the Arduino is attached and removed and it opens and closes the serial port appropriately, this means you can leave the program open all the time. The laser cutter, softdog dongle and Z controller could also all be attached to the same USB hub so you only need 1 wire running to the laptop while cutting.