NB...This is OK if pressure was safe(what condition was primer...was extraction easy?) in a modern .357(reviolver ? you don't say in your post) in good condition.
The profile/type and diameter of the bullet, seating deapth(was the W/C flush by any chance... or depressed!!!?) and a heavy crimp also will affect velocity and pressure.
Your mate's Chrono could need re-calibrating maybe? Was it far enough away from the muzzle?
Check OAL and chamber/cylinder dimensions and also barrel fouling and THEN use 3.8gn and work up....!
Recoil and blast should have quite a sting in it at 1,200+ FPS -did it? Get a buddy who knows the round to check yours out by trying a shot or two.
Pressure is the (KaBoom) "killer" - not velocity.
Tabulate all your data as you go - of course.
My 5C worth. Let us know how you go.....