1-5 - simplest mechanisms (teapots, irons, mini-droids, computers)
6-20 - medium mechanisms (wrist crossbows, multifunctional blades, traps, light and medium power firearms, lightsabers)
21-50 - complex mechanisms (Ballistae, catapults, mechanical armor, exoskeletons, heavy firearms, weapons of mass destruction, droids, starships)
50 - maximum skill level.
Well... This is, IMHO, unnecessary.