Hi,
I have to recommend applying the gap all the way across all three legs of the E. When you do that, the gap is in the magnetic path twice instead of once so you use half the total required gap. At least you dont have to grind anything that way.
You should also verify the inductance with a test or two just to make sure everything works as planned with the applied gap.
Just to note, i have gapped ferrite toroids before by cracking the core in to 4 pieces and then putting the pieces back together. It's interesting to note how it affects the core performance. The break doesnt always come out as clean as we would like it though.