Can someone explain to me what Auto Precision does and what do the settings for it mean ?
Thanks.

It's pretty difficult, it has to do with subpixel-detection.

Basically, when you zoom in, Krita has a way of detecting how much the cursor is percentually covering a given set of pixels at a given time, it will use this information to draw the brush-dab on top of it. This is useful in making very fine lines.

The downside of this precision is that, if you set it to '5', the very large brushes will become slower because it will try to calculate this precision for each pixel a brush covers. So you can set it to '1', to make large brushes faster, and small brushes less precise.

However, if you switch between sizes a lot, you don't want to change precision everytime. So hence auto-precision.

Howto use:
*First set the precision value. This can be 1, this can 5 or anything else.
*Check auto-precision.
*Set the start value to whatever the brushes current size is.
*The delta is the value that needs to be decreased or increased from the base size to change the precision.

So with precision: 5, base value: 25 and delta 5, the precision will be 4 at brush-size 30, and 3 at brush-size 35.

Or at the least, that's how I understand it.

Cool,
I think I get it, thanks for the detailed info.