Measure the thickest part of the working limb "t"
t x t x t muliplied by the width at that point will give some sort of figure for the stiffness of that part of the limb.
Take the thickness where you are going to notch it T.
T x T x T multiplied by the width after notching will give a figure for the stiffness there.
You want that figure to be a good safety factor bigger than the first figure. If it's the same or even close then it's going to try and bend.
Or just guess
Or add a shelf on rather than cutting it in.
Del