Hi Al
Without sounding uppity you need to read your chrono instructions on how to operate it as they are all different, but a few basics to ensure are:
1. Make sure it's set to units we use - so typically FPS/FPE/gn or your readings will be meaningless
2. Ensure consistency, you must shoot through the exact same way/distance/angle every time or your readings will be false
3. Don't shoot the chrono! Get it aligned and have a good backstop of open area behind it.
Pellets out the tin are fine for just casual checking of power, but if you weight ten pellets to within 0.1gn then do a 10 shot string then this is far more accurate and usable data.
And remember, most pellets will perform differently in most guns, it's common for you get to different power readings with different pellets, even the same pellets of different batches.
Typically lighter pellets perform better power wise in a springer, and heavier pellets perform better power wise in a PCP, but not always I use AA Express for springers and JSB heavies for PCPs to make sure I stay legal