I am going to provide a slightly, IMHO, easier formula. At least it is for me, I'll leave you the judge of that.
First it's easier to remember.
We know M = W*A (moment = Weight*Arm)
A = M/W (Arm = Moment/Weight - thats all you need to remember!)
A= (M +- Change in Moment)/(W +- Change in Weight) (Arm is Moment Plus/minus change in moment)/(Weight plus or minus change in weight)
For instance, if you added weight, you would ADD that weight to the bottom and add that items moment to the top (it's weight times it's arm).
If you moved an object backwards, you add to the top the addition in moment (the weight times then arm change, like from 88 to 92 is 4).
For Plus and Minus, just think of it this way. If the moment will increase (moving object aft/away from CG) or just increasing weight, you always use pluses.
With this equation, you can solve ANY problem, including your own. Sorry my online explanation isn't the best - I usually go through some exercises with my students first.