I think the answer in your example should have been -136 (as bj noted in his
reply);
the following formula (in cell K3) is a possibility.
=IF(J3/L30,AVERAGE(J3,L3),IF(AVERAGE(J3,L3)=0,AVERAGE(J 3,L3)+100,AVERAGE(J3,L3)-100))

"Jambruins" wrote:

I have numbers in cells J3:J61 and L3:L61. Some are positive and some are
negative. I would like cells K3:K61 to find the middle value between the
corresponding J and L cells. For example
cell J61 is -185
cell L61 is +113
I would like cell K61 to be +136.

Once a positive number goes down to +100 it would then go to -101 instead of
99. How would I do this? Thanks