fudge

fudge factor a figure which is included in a calculation in order to account for some unquantified but significant phenomenon or to ensure a desired result.

☞ Fudge, apparently originating in the mid 18th century as an exclamation of disgust or irritation, later acquired a specific verbal sense in printers' jargon, meaning to 'do work imperfectly or as best you can with the materials available'.