Wheel position in track-style horizontal rear forkends
Where it is now is perfectly safe. 

You can even go as far back to the blue line (where no part of the nut crosses the line).

Just want you to check whether there is a screw/bolt in the red arrow? I'm pretty sure there should be, but from the picture, it looks like it might be missing. 



In general, you want the axle to be central to the dropout, if you have it too far forward, you might find it hard to install or remove the chain to take the wheel out. 

There is also some element of changing how the bike rides by changing position of the axle. Further back is more stable in a stright line, further forward is better at cornering/changing direction. Although how it affects the rider is pretty minimal, if at all noticable.