1) How does "spacing" parameter affect real spacing between brush footprints? Seems, it isn't proportional to brush tip size (if so, circles always intersect, or always don't intersect each other), and is not fixed pixel distance (larger circles gives larger spacing). See the screenshot (same preset settings, just different size controlled by pressure): http://dobrokot.ru/pics/i2015-10-21__05-19-54_40kb.png

Thanks!

Solved - I had just to uncheck 'auto' checkbox. For values other 1.0, this 'auto' option gives unpredictable result.
When 'auto' is disabled, spacing is matching proportion of brush size.

Took 3 days to find out