Something always keep in mind is exactly how SD and ES work statistically. Most aren't aware what the actual SD numbers their chrono spits out actually mean.
To get the correct data, you'll need to extrapolate the SD using confidence intervals and the amount of shots fired. 95% is the most common interval used.
So, for example, if you fire a 10 shot string, and have an SD of 8......using a 95% confidence interval, your true SD is somewhere between 5.5 and 14.5fps. Meaning if you were to shoot enough rounds to be very sure your SD is the true SD of the ammo, it would be somewhere between those two, 95% of the time.
Now, SD itself is a distribution. Meaning 68% of your shots will be within one standard deviation. If you have an 8sd, 68% of your shots will have an ES of 16 (8 each way).
95% of your shots will be within two standard deviations. Which means if you shot 100 rounds, 95 of them would be inside an ES of 32fps (two SD of 8 each way.
99.7% of your shots will be within three standard deviations. We'll just round up to 100%.
Which means, if you have a true sd of 8, if you shoot 100 shots, you will have an extreme spread of 48fps.
If you only shoot a 10 shot group and get an SD of 8, with the confidence interval calculated, you could actually have an ES of up to 87fps.