While the fmt flags for width and precision are probably well know to most people, their
syntax is not explicitly defined in the fmt package documentation, rather only having
examples from which to derive the syntax.

In the Printing section: "For example, given 123.45 the format %6.2f prints 123.45
while %.4g prints 123.5." and other examples in the [n] numbered argument text.

In the Scanning section: "Width is interpreted in the input text (%5s means at most
five runes of input will be read to scan a string) but there is no syntax for scanning
with a precision (no %5.2f, just %5f)."

Which version are you using?  (run 'go version')

go1.2