Grapple SM question

[MagiMaster]

Ok. It seems this optional rule can neatly combine several related SM rules. Let me ask if this sounds about right.

Two important numbers here:
#1) [The opponent's overall SM] minus [your overall SM] (max +4)
#2) The hit location penalty of what you're attacking with

- When attacking, you apply the better of the two to your attack roll.
- If #2 is better (not just equal) your attack is a large-area attack and can't target specific hit locations, unless grappling.
- When grappling, slamming, etc. you're counted as attacking with your whole body, which makes #2 zero.
- Other penalties (hit location, etc) apply after this.

Does this leave any weirdness or uncovered edge cases?