Kossel mini convex print plane calibration

HI, I have bought the kossel mini 3D printer kits recently. I have now assembled all the parts, following the online instructions and get the printer set. However, I have faced a problem.

I have the extruder head stay one A4 paper thick above the bed surface for three tower positions (A, B and C). However, my D tower (the center) stay around 3mm above the surface. The problem is the printer visually calculate the print plane as convex surface.

People say you have to increment the delta radius (by the diagonal smooth rod value) to lower the extruder head so it move flat. However, no matter how I change the value (increase and decrease) it doesn't changed...

Can anyone help me?

Hi, I'm also new here =)

Have a had my MiniKossel for two years now and just the other day got the same problem when I recalibrated. Not that much but 0.2-0.3 mm. I had to increase the "diagonal smooth rod value" 1 mm to get the print head down. I don't know what the value represents. Default is 145 mm and I cannot figure out what it refer to. Some kind of radius maybe. If you haven't got it right yet I suggest you play around with the value and make bigger changes to see what happens. Good luck!