I used the narrative below to dial in my form. It comes from a progression to teach Olympic lifts. If you're not building up to learn the jerk, maybe leaning back is okay, but I found that having my head "in the hole" helps my stability.
"The military press begins the overhead area and prepares the athlete for the jerk. The athlete begins by standing with their feet hip width apart (jumping position) and have the bar resting on the front portion of the shoulders similar to the front squat position. It is different from the front squat position though in that the elbows are rotated up just enough to let the bar rest on the shoulders, they aren?t all the way up to parallel and the bar is held with the whole hand rather than just two fingers. From here the athlete will simply press the bar overhead till the arms lock out. The finish position is critical here. The athlete must get to what is termed the ?head through the hole? position. The ?hole? or window as some call it is the imaginary window that is outlined by the shoulders on the bottom, the bar across the top, and the arms on the side. The head must poke through this window. The finish should have the bar directly above the shoulders and even with the back of the ears. This is a must once we advance to the push jerk and jerk because the athletes will eventually be able to drive much more weight over their head by using their legs than their arms can hold. They will need to have that bar in the slot or head through the hole position in order to balance the weight on their joints. "