Kossel Delta Z axe calibration

Calibrate Z axe with setting perfect distance from top for all tree towers.
Three steps:
- Set your rod length (by measuring rods)
- Change diagonal (horizontal) length until print is not neater convex nor concave (useful tips on http://letsmakerobots.com/content/kossel-mini-calibration)
- Change screw distancers of each tower (that touch endstops) until you get same line width on all sides