I shot 34 rounds today and got an SD of 4.0
6 dasher in Alpha brass
I am more concerned with ES
Then you need a class in statistics. Standard deviation will tell you the expected spread, 98+ % of rounds fired will be within +/- 3 SD's of the mean. ES is more or less useless. Fire 5 rounds and take the ES, how many data points is that? 2. Fire 100 rounds and take the ES, how many data points? Still 2. Now, with that said, if you fire a string and the extreme spread is higher than 6 times the SD you are looking for – STOP. That load isn’t going to get you there. ES can be used to reject a load, but it will not reliably help you choose the right one.
20 rounds will provide a mean MV and an SD that has a 90% confidence interval. If your recorded SD is 6 fps, your true SD is probably between 3 and 12 fps.
Unless you are shooting ELR type ranges, say 1200 yards and further, SD, unless it's huge, like 25fps or more, is probably meaningless as well.