Bob,
With a rifle new to me, I always test on two (2) different 'reference' type chrono's - usually a SKAN and LMBR R2A.
I too have found that all my chorno's agree within reasonable limits.
To get a reasonable estimate, you will need to chrono' at least thirty (30) shots, and have a good idea as to the average pellet weight (weigh 100 pellets, and divide weight by 100).
Calculate the hypothetical maximum velocity : mean and standard deviation (SD) of the thirty (30) shots, then the mean + (3.3 X SD).
Then work out the hypothetical maximum energy from hypothetical maximum velocity (HMV) and weight of pellet :
( 'pellet weight in gr.' X HMV X HMV / 450660)
The value 450660 is g X 7000 X 2, where g is acceleration of free fall, and is ca. 32.16 f/s
2 where I live.
This hypothetical maximum energy is unlikely to be exceeded in 99.9 % of times.
For me, this hypothetical maximum should be 11.5 ft.lb or less.
Have fun & a good weekend
Best regards
Russ