Every gun has a different 'leade', the smooth barrel part before the rifling starts. Some have a longer leade while my CZ's is very short.
A dummy bullet is made using a case with no primer or powder, and the bullet is seated to various depths to determine at what point the round will chamber without sticking.
Take the barrel out of the gun. Seat the bullet to a length that fits in the magazine comfortably. If the round drops in the barrel with a nice CLINK and no sticking, you have your proper depth. If not, seat a tad deeper until it does. Be careful though, the deeper a bullet is seated, the less your powder charge can be to avoid blowing up your gun and hand.
You'll also have to verify it'll cycle properly as some bullet profiles aren't agreeable with all feed ramps and chamber types. SWC bullets have this issue.
On a side note, I bought a Lee mold, 125gr flat point for my 9mm. It has a weird shape (or ogive) and my CZ wouldn't accept it until...1.02oal. It was so deep I had to back off the powder charge to minimum levels so it wouldn't blow up.