If you have a custom built delta or even a machine that was designed for you it's important to make sure that as the hot end travels from the center to the extremes that it travels flat. In this tutorial I will attempt to explain now I set the flatness of a delta printer.
1) Make sure your towers have adjustments on the carriages so as to adjust the distance. Because mine didn't have any I ended up printing a block that sits on top with a screw that I could turn for the adjustment. There are some printable carriages that have the adjustments built in on thingiverse if yours dont have any.There are also bed holders that will let you adjust the height and that can achieve similar results. I like to leave the bed stationary and adjust from up top.
2) Make sure that the distance between the tower end stops are about the same distance from the top. You can wedge anything in between the top of the frame to the end stop with something you have found lying around or use caliper. You dont have to be perfect here but try to be a close a possible from the top of the fame to the ends stops on all three towers.
3) Make sure your carriage adjustments are approximately all at the same level
4) Set the Z height. (G0 Z8 F8000) from, there manually go down till you get to 0. Adjust your MANUAL_Z_HOME_POS in firmware until you complete the paper test.
5) Make sure your carriage adjustments are all at the same level and then probe each corner. I use Rrepetier-Host for this. Right click on the script buttons below the manual controls in order to bring up the script window.
This is what Script 1 looks for me below. Your X and Y position will vary so don't copy it exactly. Manually move the hot-end using the controls until you get a position that is close to in front of the first tower you are working on. You don't have to go to the edge of the build plate but I think a little more than half from center to the tower is good. Record the X and Y position and substitute that for the X and Y in the script.
; Adjust X Tower
G1 x-80 y-40 z4 F8000
6) Do the same for the remaining towers to set your scripts. I have mine set to 1 being X tower adjustment, 2 being Y, 3 being Z, and 4 is the script below which will just bring it down the center. The "F" number is your speed.
; Adjust Center Tower
G0 z4 F8000
7) Once you have your scripts tap the 1st one to adjust that tower. Do the paper test on each tower. As you adjust the towers the center height could change to the point where it might be lower (crashing) or higher from the build plate and that's ok. This is where we will set the flatness of the movement.
8) Put a laser level on a tripod and set it mid way to a 1/3 down from the bottom. jog the hot-end down until the laser is hitting the hot end tip or sensor. Anything that is easy to track and fixate on. Make note of the distance so that you can reuse the script.
9) Pull up the firmware and look for DELTA_SMOOTH_ROD_OFFSET. You'll be adjust that number up or down.
Increasing DELTA_SMOOTH_ROD_OFFSET lowers the hotend.
Decreasing DELTA_SMOOTH_ROD_OFFSET raises the hotend.
10) Set the number really high and upload the firmware. Connect in R-Host and go down to that pre-determined height that lines up with the laser. Next jog the hot-end to the corners to see whether the hot end dips down or rises up. G28 it to bring back up.
11) Go back into the firmware and set back the DELTA_SMOOTH_ROD_OFFSET to its original number and upload. Perform the test by jogging the hot-end to a corner and see if it rises or falls. G28 it to bring back up.
12) Go back to the firmware and make your adjustment and upload. Move the hot end until it travels flat with the laser. G28 it to bring back up and repeat until you have something that is acceptable.
13) Set the Z Height again and do the paper test. G28 it.
14) Set the height to each of the towers using 1, 2, and 3, scripts that you created before. Complete the paper test and make your adjustments on the carriage end-stops for each tower.
15) Repeat all steps until all points on the tower corners and the center pass the paper test.
I would like acknowledge the author of this link, Thomas Brittain, for providing the technical insight that inspired me to write this tutorial.