For your 20 feet per second test, as you said, set up your optical chrono vertically and place a lamp directionally across the sensors. Take a 150grain bullet at around 6.2 feet from the first sensor and drop. If my math is correct (and I damn sure wouldn't take that as a given) the bullet should reach 20fps in between the sensors.

The MagnetoSpeed V3 and the Garmin Xero C1 Pro are the 2 I have. Dad used Chrony's for Bow primarily. He used published data for his reloading speeds since his primary concern with reloads was grouping accuracy. Not to mention they really didn't have all these fancy whoopdi doos when he was still with us. Light sensor only.

Oh, I'm fairly certain you know this already but in the slight chance you have never heard this, try your best to go through the center of your light sensor chrono's. High or low across the sensors will indeed cause variations. Significant ones. I have noted some as much as 60fps variance between a center crossing and a low crossing. Also, hitting the chrony is bad juju. Lets not discuss how I know this.