A fid is a splicing instrument, but the length of a fid for a given cording is a multiple of the diameter of that same line (21). That's why splices are listed that way...in relation to the cording itself.
Here's a chart of some common cordage used by members of the forum, and the lengths that whoopie buries and eye-splice buries should be (per the manufacturer's safety ratings).
BTW simonx23 - You probably want to use 13ft of cording for 6ft slings. Some of the length will be lost due to the buries and backsplices.