Clay pigeon shooting
The problem with this target is that as it loses momentum at the top of its climb it can easily be knocked off line by even the slightest of breezes – hence your problem trying to determine its apex.
And when there’s a proper wind blowing the whole thing can turn into a nightmarish situation.
The other problem is that even on a relatively still day the clay doesn’t fall in a consistent manner so shooting it on the way down often becomes a very ‘iffy’ hit and miss affair.
To avoid this my advice would be to shoot it on the way up while it still has sufficient power to hold a consistent line and that’s just before it loses momentum and comes to a stop.
You probably don’t need reminding this bird is easy to miss over the top so don’t hold the gun too low when calling for the target as this could encourage you to catch up by swinging too quickly and ‘overshoot’ the mark, so to speak.
Instead, hold the muzzles fairly high and INSIDE the clay’s line if it’s angled in any way.
By holding high you can now watch the target all the way up as you move and mount the gun.
Don’t let the clay get passed the muzzles, keep the gun moving and shoot just after the mount is complete.