I just did the same tests. After unequipping the Vim Crystal, I did 12 damage; with the Vim Crystal and 7 Stone's Throws, I did 21, for a total +9 increase. (Did you unequip the Vim Crystal? It's equipped when I load the save, and I initially forgot to unequip it and landed a 14.)
The current formula goes [Wall Defense * Number of Stone's Throw effects / 4 ] before rounding. So in this case, we have 4.5 * 8 / 4 = 4.5 * 2 = 9. (Without the Crystal, it'd be 3 * 8 / 4, for a +6 increase,)
Now the question is, do I change the formula? If I divided the wall defense by 4, rounded that up (for +2 per Stone's Throw in your case), and then multiplied by Stone's Throw count afterward, we'd have 2 * 8 for a +16 bonus. Of course, a +4 Rock Wall would end up with the same bonus; there wouldn't be any difference between them unless we start adding other items into the mix.
I'm hesitant to change it as it's already a large boost to Attack when stronger Earth Formulae (like the Super Powder of Rock) get involved.
