Upper-chest flys with cable tension all the way.
Keep the elbow bend fixed and sweep wide. This is a fly, not a press.
Setting an incline bench between low pulleys angles the fly upward, which lines the resistance along the upper chest fibres that flat pressing tends to leave behind. The cables resist from the deep stretch all the way to the squeeze above your chest, so there's no slack point at the top like there is with dumbbells. It earns its place for anyone whose upper chest lags, offering an isolation angle you can hammer without adding more pressing volume.
Around 30 to 45 degrees. Lower keeps the work closer to your mid chest, steeper starts handing the movement to your front delts, so if you feel it all in your shoulders, drop the angle.
Usually the bench is too steep, the weight is too heavy, or your arms are drifting into a pressing path. Reduce all three, fix a soft elbow bend, and think about hugging a wide arc rather than pushing the handles up.
Whichever matches your weak point. Flat covers the chest more generally, the incline version deliberately biases the upper fibres, and if you only fly once a week, most lifters get more from the incline.
