Roy,
I've never heard of having to calibrate a chronograph before, but I realize there may be some out there that possibly could have to be.
I have a very good, accurate chronograph that I've been using for many years. I've used it for everything including testing reloads, compound bows, trad bows........you name it, and I've probably shot it through my chrono.
The biggest thing about getting accurate results is that you must be consistent with how far you draw each arrow when comparing speed results of various bows. Makes a big difference. If you're getting very erratic readings from one arrow to the next, then something is wrong somewhere. Either you don't have the proper lighting, or your arrows are way off in weight from each other, or you're draw length isn't very consistent or something. Many people tend to overdraw their bow when shooting through a chrono, sometimes even without realizing it.
Also, I like to get back only about two or three feet from the chrono when shooting bows through it so I can know exactly where my arrow will be passing over it and also so I won't hit the chrono when I shoot!