HTML Table column width practices

What are good practices when selecting column widths in a table? Let's say I have four columns, name (variable width), description (long content of text), count (max 3 chars), date (fixed format).

What would be a good practice? I'm thinking fixed width for descr., count and width (thus actually also making name "fixed" width). But my real question is, how to select a particular width size. For instance, if the date format is yyyy-MM-dd is there some trick to convert those 10 chars to a width which will guarantee that it shows ok in any browser using any font and font-size (without also taking up any excessive space)?

edit: With fixed I mean something akin to "fixed amount of pixels relative to font width"