The way I look at it.
Every formation has a unique setting for closing down to work optimally.
The defensive line essentially makes the midfield and the defense work together more effectively as a unit, but only when not in possession of a ball.
Setting Ultra attacking mentalities up front and ultra defensive mentalities behind has the effect of stretching your team, which means when you win possession more often than not its a punt or a great thru ball, never a build up through the centre.
Once you have set closing down instructions, the optimal dline should be normal. At this DL if your closing down instructions are not right, then you will see the opposition get more of the ball in midfield. But if you do its a 50/50...to see this you HAVE TO WATCH IN FULL for a while.
Once the closing down instructions are optimal, that means your midfielders are doing a good job of winning the ball and your defenders go to the 25 yard line to close the ball out then you..have the right settings.
Now to turn on the screw...you increase DLine this has the effect of making your mids try to win the ball further up when not in possession....this is where things get complicated for everyone...to avoid confusion, think of the whole team as one unit. The space it occupies is one space. DLine just moves an imaginary line between the Mids and the defenders further apart...Mentality governs how long that horizontal line should be.
Width has the effect of making you more compact but does not affect how long you can be in mentality terms.
Now that your DL is set...if you want your players to ease up...reduce, when you want to chase a goal you push up...to see this more clearly watch the game IN EXTENDED HIGHLIGHTS...you should see yourself winning more and building more attacks...watch your possession stats...they may even increase especially against defensive teams, but may go down against good teams...i say may not will.
I do not use Rule of One or anything like that the game is fundamentally very simple...Mentalites can be adjusted but too high the gap between players too easily possession will be lost, which is why some people have success with ONE MENTALITY.
Remember the thread a couple of months ago which said that you can play defensive mentalities all the way and you can still be successful?
The best tactic to see this in operation is any 442 based tactic. However Closing down instructions vary from formation to formation. I do not believe one moment that you need to stand off and allow a team more possession when away from home...you can do that easily by drawing them in with the DLine...you can even make it harder for the AI by going narrow.
Remember if you play with a deep defensive mentality player and you tell him to close down often and if the whole team is in your half...you could find him straying....if you set it to high for a winger..the winger will wait till its in his zone...will track a player may even tackle, you may even get a 2 on 1 if the AI enters another players zone.
DLine is simple and easy to use. Its the only tweak you need when going for goals.