Distortion calibration on XY plane in Repetier
Skew distortion in deltas means there is something physically wrong with your printer build, such as the towers not being evenly spaced or being tilted. The first thing you should do is confirm the mechanical build -- measure the distance between towers, angles between towers, parallelism of all three towers, and perpendicularity of all three towers to the bed. If you post photos of your build, we might be able to provide more specific advice. 

If you can't get the mechanical issue sorted, it's possible to calibrate out some specific build errors (like skew due to uneven tower angle), but that's nearly impossible to do "by hand." You really need to use a Z probe and auto-calibration sequence such as in Rich Cattell's Marlin fork or dc42 RepRapFirmware.