First layer uneven  

First layer uneven

I´m sorry for posting this,  it has shurely been discussed many times on this forum, but I couldn´t find a similar question:

My first layer is extremely uneven, and when I do a print with a big surface, it looks like on one side, the nozzle is too far away from the bed, on the other, it is to close.

(Image shows a first layer of an 170x170 sheet)

Some regions are absolutely perfect, others are just 40microns (0,04mm) thin, and again others have about 600 microns (0,6mm).

The first Layer was set to 0.3mm, Print sheet is textured PEI.

Filament used: Jet Black Prusament PLA

My mesh bed levelling is kind of working, when i pus a piece of filament under one edge of the steel sheet, it follows its contour, but not good enough (at least in my opinion).

On my other printer, running RepRap, I can set the number of probing points etc, to get a finer mesh. Is there a way to do that on the mini?

Thanks for your help!