fudge factor

fudge factor n.  A value or parameter that is varied in an
   ad hoc way to produce the desired result.  The terms `tolerance'
   and {slop} are also used, though these usually indicate a
   one-sided leeway, such as a buffer that is made larger than
   necessary because one isn't sure exactly how large it needs to be,
   and it is better to waste a little space than to lose completely
   for not having enough.  A fudge factor, on the other hand, can
   often be tweaked in more than one direction.  A good example is the
   `fuzz' typically allowed in floating-point calculations: two
   numbers being compared for equality must be allowed to differ by a
   small amount; if that amount is too small, a computation may never
   terminate, while if it is too large, results will be needlessly
   inaccurate.  Fudge factors are frequently adjusted incorrectly by
   programmers who don't fully understand their import.  See also
   {coefficient of X}.