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 which 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. An example might be the coefficients of an
       equation, where the coefficients are varied in an attempt to make the
       equation fit certain criteria.