I will tell you my process for my pistol reloading as to what velocity range I look for. First you need some sort of base line data or standard in which you can measure what you produce.What do you consider acceptable maximum spread in velocity for the various rifle and pistol calibers you load for?
How many rounds do you fire to get an average? I've heard everywhere from 3 to 20. What say you?
What's the best you've achieved?
Lets take a popular caliber for an example - 9mm fired from my G17 w/4.49" barrel. First I purchase 3 boxes of factory ammunition (124g FMJ RN) to obtain a base line for my reloading data. I fire 6 rounds for an average. I feel 6 to 10 rounds (max) is more than sufficient to obtain an accurate average.
Federal "American Eagle - 1,044.6 fps
Remington (green.white box) - 1,098 fps
Winchester USA (white box) - 1,039.1 fps
take the average of these - 1,060,5 fps / now you have what the average velocity from what the commercial ammo people are producing. In this case for your general target use I now have a "target" velocity to start with. Now you just decide for your self based on your own test results what is "acceptable" to you.
In this example the accuracy will dictate the final outcome but I will try to keep it in-between 1,040 & 1,080 fps