Print 3D 
The automatic calibration for delta is done in the DC42 fork of the RepRap firmware. It shall be noted that this fork have been adopted by Think3DPrint3D for the Duet 0.8.5 and by RepRapPro for the Fisher delta, however the development is done at fast pace and the fork is always ahead of the 'official' release of RRP and T3P3, particularly regarding the calibration. So, some comments and parameters below only apply to the original DC42 fork version. (development branch). As 19 October 2015, the last version of this fork is 1.09k.
If using the Fisher, for good calibration, the calibration macros delivered with the Fisher shall
Number of parameters (Sn)The calibration use defined test positions and adjust parameters according the
(according David Crocker) Six parameters calibrationThe 6 parameters add to the 4 parameters test the angular position of the X and Y columns (they search if the triangle is not equilateral), as this is a current default on "Kossel type" printers. This is why there is only X and Y angle, the Z angle being the reference angle.
The problem is that the calibration algorithm attempt to transform all these defaults in angular position fault. Generally speaking, the algorithm find a solution with angles which minimize the hotend/bed delta, hence helps to have a nice first layer. These found angles may be way off the reality.
On the Fisher, it is reasonnable to consider there is no angular fault.
For other printers, you shall print delta calibration part or this part to research your angular faults, then enter the corrected angles in the M665 command in the configuration file (or in the Gcode windows, but this is lost at restart). After enter of the corrected angles, reprint a new delta part till finding the good angle. Then use 4 parameters calibration. There will be a larger delta between the hotend and the bed, but your parts dimensions will be more accurate. Number of measured pointsAccurate results are only obtained if you have at least 3 or 4 measured points more
Measurement ZoffsetOn the Fisher, the measure of the bed position need force and create a measurement offset because
ExamplesYou will find below extract of my own Fisher calibration macros, but it is only supplied
Extract of my bed.g macro, with 7 measurement points and 4 parameters ; Probe the bed and do auto calibration G1 X64.95 Y37.5 F12000 G4 P300 G31 Z0.12 G30 P0 X60 Y37.5 Z99999 ; X tower G4 P300 G30 P1 X64.95 Y37.5 Z99999 ; Y tower G4 P300 G30 P2 X0 Y75 Z99999 ; Z tower G4 P300 G31 Z0.15 G30 P3 X32.48 Y18.75 Z99999 ; half way to X tower G4 P300 G30 P4 X32.48 Y18.75 Z99999 ; half way to Y tower G4 P300 G30 P5 X0 Y37.5 Z99999 ; half way to Z tower G4 P300 G31 Z0.17 G30 P6 X4 Y0 Z99999 S4 ; centre, and autocalibrate Extract of my main calibration routine, with 10 measurement points and 6 parameters ; Probe the bed and do auto calibration G1 X64.95 Y37.5 F12000 G4 P300 G31 Z0.15 G30 P0 X60 Y37.5 Z99999 ; X tower G4 P300 G30 P1 X64.95 Y37.5 Z99999 ; Y tower G4 P300 G30 P2 X0 Y75 Z99999 ; Z tower G4 P300 G31 Z0.18 G30 P3 X32.48 Y18.75 Z99999 ; half way to X tower G4 P300 G30 P4 X32.48 Y18.75 Z99999 ; half way to Y tower G4 P300 G30 P5 X0 Y37.5 Z99999 ; half way to Z tower G4 P300 G31 Z0.22 G30 P6 X52 Y30 Z99999 ; between X and Y towers G4 P300 G30 P7 X52 Y30 Z99999 ; between Y and Z towers G4 P300 G30 P8 X0 Y60 Z99999 ; between Z and X towers G4 P300 G31 Z0.20 G30 P9 X4 Y0 Z99999 S6 ; centre, and autocalibrate Simpler calibration routine.On recent versions of the DC40 firmware fork (check: present version is k), a parameter to indicate the offset for each probe point was added. That makes the routine more simple and readable.
M201 X500 Y500 Z500 ; reduce acceleration ; Probe the bed and do auto calibration G1 X64.95 Y37.5 F12000 G4 P300 G30 P0 X60 Y37.5 H0.12 Z99999 ; X tower G30 P1 X64.95 Y37.5 H0.12 Z99999 ; Y tower G30 P2 X0 Y75 H0.12 Z99999 ; Z tower G30 P3 X32.48 Y18.75 H0.15 Z99999 ; half way to X tower G30 P4 X32.48 Y18.75 H0.15 Z99999 ; half way to Y tower G30 P5 X0 Y37.5 H0.15 Z99999 ; half way to Z tower G30 P6 X4 Y0 H0.17 Z99999 S4 ; centre, and autocalibrate M201 X8000 Y8000 Z8000 ; reset acceleration  supersede values set in config.g Results2 iterations of 10 points/6 parameters calibration done after a cold start and
Note:Result is not exceptional, this was the only trial done to write this web page. 1 iteration of bed.g (7 points/4 parameters) after a cold start and hotend heated
