Isolate the upper chest with zero balance worries.
Keep the soft elbow bend fixed. Hug the weight in, don't press it.
The machine's fixed arc sweeps your arms together along the upper-chest line while the seat supports everything else, so the pecs get isolated with zero balancing demands. Because the machine keeps resistance on through the whole arc, the stretch at the open position and the squeeze at the peak both stay loaded, with the front of the shoulders assisting through the sweep. It earns its place as the safest way to train the fly hard, letting you chase a deep stretch and take sets close to failure without a spotter or wobbling weights.
So the handles line up with your upper chest and your elbows sit just below shoulder height at the start. Too high and the shoulders take over, too low and the arc misses the upper-chest line the machine is built for.
As far back as you can control without the shoulders complaining, and pause there briefly. The loaded stretch is half the value of a fly. Just move into the deepest range gradually rather than diving to the machine's end stop on rep one.
Usually the soft elbow bend is changing. If your elbows bend more as you fatigue, the fly quietly becomes a press and the chest unloads. Fix the bend angle at setup and keep it identical from stretch to squeeze.
