The Fl_Value_Output widget (amongst other valuators) can be used for decimal numbers. This particular one is just for output. The format it uses for the number is determined by the step() method -- you don't have complete control. The smallest useful step is 1e-9 (giving nine decimal places). If you try for more, the format changes to 5 places.
If you need a very specific format, the best thing to do is to generate a string having the number correctly formatted (using sprintf, or ostrstream ) and display it in a Fl_Output as text.
MSC 9 Dec 2005