Hmmm, Good question as everybody has their own method and distances. Listen to GLP above me here. I have learned by experience that what he says is true.
I shoot the AAFTA Grand Prix matches here is the South East and found each gun and each FPE has their own variables in where to zero. I zero my 20 FPE Steyr at 30 yards, A Crosman Challenger that I bumped to 12 FPE at 30 yards also.
I have an HW97K that I zero at 27 yards and My comp pistol, a Crosman 1720 T, that is bumped to 18 FPE at 24 yards. I get to these by first plugging in the information into Pellet Path a program you can download to your phone from Hector Medina's web site at Connecticut Custom Airguns. After sighting in I take a center cross hair shot at a 1/4" dot at 10 yards. Then I do the same at 55 yards at a 3/4" dot. Each time I measure down from the dot I aimed at. to where the pellet actually hit. Then record the spread between them in inches. I plug that into the pellet Path program on my phone and hit enter. Then the pellet path will calc the clicks or mildots, in the form of a chart you can transfer to pape,r or as most who use it leave it on the phone and carry it like a holdover chart.
I go one step further as I have the Eric Sanders oversize elevation Knobs and side wheels/nautilus on my guns. He has a program for the laptop called Scope Knob. I use it for my final calculations, as it also prints out my elevation knob in clicks and the side wheel tape. Much neater and easy to read.
I just put the same difference in inches at 10 and 55 that I put in pellet path.
. His program will calc the clicks for me as I shoot open or WFTF class in field target.
The key for Hunter Field target is to not have any hold under's on your holdover chart. Very confusing otherwise. The two programs will let you know if their is hold under's when you plug in your distance, scope height, pellet speed, etc. You can change your zero in the program until it gives no hold under's. That will be the zero you want for that gun.