TargetTalk

It seems that I have too low prone position - too close to 30 degrees or might be even more - it tends to saddle.
What should I do generally to raise it up?
Move the handstop forward and shorten the sling for the same lengths?
Should I lower the buttplate? And/or change the slingholder position? (I have it quite high - close to the armpit)

I had quite a hard time to find a comfortable position and afraid if I change one parameter - then will need to start searching from the beginning..

You may be able to keep yor basic set up just increasing the angle of your forearm probably by shortening your Handstop slightly. This may result in altered sling tension and butt height slightly.

Moving your handstop forwards, and shortening the sling is probably not a good solution. Yes would raise your forearm, but it would also increase the the pressure of the butt in your shoulder. Unless your position is a bit too loose now, the extra pressure would probably be too much, so your shoulder becomes tense.

As Martin suggested try moving the handstop back a touch, and shorten the sling.