Yeah, it's hard to do on those desks, I ended up selling mine lol
The closest you can get is probably putting the monitors on stands right next to the metal poles and using a keyboard tray so you can sit 40cm further back.
I guess it's no biggie really, just sit like 80cm back from where you are atm when youre listening critically. You won't be able to reach your keyboard etc. though so mixing will be a bit of a bitch - have to keep sliding back and forth to check stuff.
The higher frequencies are, the more directional they are. The lower they are, the less directional they are. By "directional" I mean your ears being able to distinguish where it's comming from. SO, with that in mind your ears should really be in line with your tweeters, or maybe just a tiny bit below. I don't really think thats a huge issue though, since you're going to slouch/sit upright/slouch/sit upright etc. and youre constantly going to be changing position lol.
With the angling down I would be more concerned about the phase alignment of the speakers, since the highs will reach your ears slightly faster than the rest. I know this is why it is bad to place your speakers horzontally, but dunno what effect that has tilting vertically. Maybe someone can clear that up
If it's bad, you can just adjust the height of the shelves
But, looking at it now, the angle doesn't look significant enough to make any difference.