BM: That is true for the most part except that you might have one or two rounds that are departures from the norm (depending on the number of rounds you use for a string) and inflate your extreme spread. After many years of doing this, you probably have a ratio built into your head that allows you to approximate an SD.
Myself, I do a lot of pressure calculations (hydraulic and sound) and after a period of time, some of the established values start imprinting themselves on your brain.
NavArch: I'm extrapolating an occupation from your handle here, but is it possible that when you calculate %RSD you are working with established values of materials, and other known physical values? Take heart, most of the meaningful research in chronograph technology over the past three decades or so has been done by Dr. Ken Oehler, and I believe his doctorate is in mathematics, if not Physics. Plus, he's a Texan!
If you have a need to know average deviation as you go along, there are chronographs that will provide that for you. The chronograph has to set the mean and then compare values to it.
I think Mal H makes a good point, or maybe what I'm taking from it. Say you're shooting groups while chronographing. Personally, I don't want the values during a string of fire unless something departs from the norm, otherwise, an apparently odd value may affect the norm itself as far as concentration and group size.
mek42: that notion would be pretty handy wouldn't it? Unfortunately, many of the powder companies just as soon you not know the SD characteristics of some powders, it might effect sales. Ramshot is the last company to have done this and if you see me rambling on about some of their powders, that's part of the reason why.