I have a forumla I want to calculate.

There are three cells that have a total percentage possible of 100% just adding them creates results different from what I'm wanting. Here is an example:


F5 = -30%
F6 = 35%
F7 = -35%
F8 =sum(F5:F7) and it reads -30%

What I would like for it to read is -65% to show that out of a possible -100% to 100% the total was -65%

Does that make sense? If so how do I accomplish this?