15 feet has become a standard used in a lot of manuals and also magazine articles. One fellow on the board said his .338 Lapua magnum needed 18 feet before readings settled. A lot of folks fire pistol as close as six feet, but I wonder about some of their readings and think 10 ft is a better minimum for handguns. I use 15 for both rifle and pistol just to be consistent.
For .38 Special barrel length is also an issue. A lot of the snubbies seem to have pretty wide velocity spread, but once you get to 3" and longer, it's not normally a big problem.
The CED chronograph page
has a good list of factors that can go wrong in chronograph readings when you pull the scroll bar about half way down, under "Variables that will affect results. . ." Some of it just applies to their machine, but most apply to all. They state that chronographs can disagree by as much as 8%.
CED's unit gets high marks for accuracy, as does the Oehler. For others you need something to compare to. .22 LR Match ammunition is usually within 50 fps of the claimed velocity. The standard velocity barrel length is 24", but the bullets cease accelerating somewhere in the range of 19", so velocity doesn't change a lot between there and 24". Anything between 18" and 24" should be close.