If your I&E includes a certain amount of overtime already you should operate on the figure listed as your net income on the I&E. Keep anything up to 10% over and above this figure and split the rest 50/50.
Don't know if anyone else mentioned it but the 10% is 10% of your normal MONTHLY pay not yearly, there was confusion a while back where people thought it was 10% of yearly pay when calculating the up lift for a yearly bonus.
Still think it penalizes those of us who get yearly bonus's where people with monthly bonus get to keep a lot more but that is a whole new can of worms...
If life is what you make it, I must have been in a strange mood when I made mine