You can mapping your problem into diagram below:

your formula is
=IF(L6-K6<0,(L6-K6)/(K6-J6),IF(L6-J6<1,(L6-K6)/(K6-J6),(L6-J6)/(K6-J6)))
where
L6 is Actual December 2011
K6 is Plan 2011
J6 is Actual 2010

diagram is