try practice mode to watch your monitors stereo field... I found that way that my speakers was not in a great position...so it help me fix the main problem first and after that it's a mater a practicing and ¨visualise¨ the sound...

it is SUPER hard for me in headphones, not sure if you're using those but i can do it just fine with speakers. The secret is practice practice ppractice :)

With headphones, it seems more difficult to perceive the distance of the sound between the phantom center and the left or right speaker.
Is this due to a lack of natural reverberation in the room, as the sound is not heard through the speakers?

thanks guys. I practice with the monitors instead and it's really better.

it makes me think... is it a big problam to mix space right using headphones?

Unless you have some incredibly sophisticated headphones, they operate on the principle of volume. Louder = closer, and this is how they trick the brain into directionality. However, the brain uses another feature: timing, where sound hits one ear before the other. Because headphones can only reproduce one of these two, they make panning more difficult to calculate over speakers.

Benjamin, that was a great explanation. Thanks