I'm using 1 chrono and moving it downrange. So you can't compare shot 1 at the muzzle with shot 1 downrange and get an accurate BC figure - When I'm quoting BC's I'm going with the average MV against the average downrange.
Not a great way to do things, but for what I'm trying to test it doesn't matter too much. It was less hassle than calibrating my combro against my pro-chrono and then trusting that taking a shot with the combro fitted wouldn't alter my poi ( and cause me to hit my pro-chrono
I'm not really trying to measure BC accurately, just demonstrate that consistency at the muzzle doesn't necessarily mean consistency downrange really.
Not really laboratory conditions I know, but hopefully better than our usual subjective tests