To properly enable MatterControl Software Print leveling on your Kossel Pro, follow these steps:
1) Home the machine and note the home Z position
2) With the probe not deployed, manually jog the nozzle down until you're pinching a piece of paper to the bed - note the Z position
3) If this Z position is >0 (and it will be), subtract it from the home Z position. Then add 2mm. This will be your new home Z position.
4) In the firmware, find the line "#define MANUAL_Z_HOME_POS" - and put your new Z value here.
5) Flash the firmware
6) Change the print area in MC to 200 x 200 or 220 x 220 (you'll get more reliable/precise readings when your arms aren't close to horizontal)
7) Run 13 point auto leveling
8) Once finished, check your values to ensure there are no negative Z values - you can do this by clicking the pencil icon next to Software Print Leveling in the Options tab in MC
9) If there are no negative Z values - you're ready to print. Change your print area back, make sure G29 is removed from your start g-code, and give it a go.
SUPER IMPORTANT - Once you've done the above, do not run your machine without leveling turned on. You added 2mm to the Z home position, so without leveling enabled, you run the risk of crashing into your bed and breaking your build plate.