Most principled variation

 The most principled goal where you can strive for in the middlegame is the assault of the king. This deliberately puts the second goal on the back burner. The second goal is to get a profitable pawn structure for the endgame.

60% of the games are decided by mate and 40% by endgames. In order to don't make things too complicated, I think it is justified to ignore the second goal for now. When there is complete  understanding of the assault of the king, it is soon enough to have a look at wat is left on the back burner.

This means that I can say that the middlegame is about the battle for the pivotal squares.

Fiddling around in the opening

Now that the goal and the method of the middlegame is clear, I have an endpoint for the opening. So I'm going to fiddle around a bit in order to get more clarity about the methods of the opening. In no particular order:

White to move

r2q1rk1/pbpn1ppp/1p1ppn2/8/1bPP4/2N2NP1/PPQ1PPBP/R1B2RK1 w - - 4 9

There is a battle going on for e4.

White can bring to following to the table:

• Nc3
• Nd2
• Qc2
• Bg2
• Re1
• Bg5

Black:

• Bxc3
• Bb7
• Ne4
• f5
• d5
• Re8

Qc2 was designed for a3 Bxc3 Qxc3, thus getting the bishop pair at the cost of a few tempi by the queen and a3. Black is ready to take on c3, play Ne4 to win another temp at the queen and f5, thus blocking e4 forever.

Black has adopted a hedgehog setup, which is full of tension.

It is a matter to get a clear picture of everything that is going on. The pivotal square e4, the used tempi, which pieces battle for e4, which pieces waste their time. What happens when the center is blown open, who is going to profit from the lines of attack (LoAs)?

I'm searching for a systematic approach which helps me to judge which moves fight for the principled variation, which moves ignore that, when to move a pawn, when to move a piece.