I'm tired of calibrating my Folger Tech 2020 Kossel using z-probe. Tried both - Marlin and Repetier. They calibrate, but I didn't like the final results. The stock Z-probe is too far from the tip (X-Y offset) and even slight effector tilt return very inaccurate values.
So, I decided to make my own adjustable hotbed mount using spare screws from printer kit (3 M3x25, 3 M3x15, 3 Nylock M3 nuts, 3 washers and 3 M3 t-nuts for 2020 extruded profile), simple sketchup-made mount (printed using PETG) and 3 springs that I found in $4 Homedepot spring kit (http://www.homedepot.com/p/Everbilt-Spring-Assortment-Kit-84-Pack-15642/202045461).
What a difference! It took only 15 minutes to calibrate the installed bed with very high precision.
Tip: Use small drill bit (1.5mm-3mm diameter) instead of paper for initial calibration - it will give you some room between the nozzle and bed and will prevent head crash. Measure it's opposite (non-working) end (let's say it's 1.98mm), set Z height to 1.98mm, move the head as close to the X calibration screw as possible and adjust it so the drill bit will roll just slightly touching the tip. Repeat for Y and Z. Repeat the procedure, if needed, then start adjusting delta radius. Repeat with paper test to fine-tune, if needed.
Finally, I dismounted the probe (here is the replacement mount to add extra fan http://www.thingiverse.com/thing:1123295) and will never use it again... hardware calibration rules.
PS: to avoid mess with slicer and make smooth transition during printing from hex-nut hole to M3 I designed the thin wall, so short bridges over hex-nut hole will do the job pretty well. You need just puncture this thin wall using small screwdriver before inserting M3x25 screw.