Here is how HQ works:
+1 adds 5% extra to each STAT WITH THE STAR NEXT TO IT ONLY. This would be: your attack, acc, and crits on your bow/all weapons. Also its defense, magic defense, and evasion on all eqp. (including rings>.<) Also craftsmanship, magic craftsmanship, and control on all tools.
+2 adds 10% extra
+3 adds 15% extra

Example :

NQ Crab bow:
127 attack

+1 Crab bow:
127 attack x .05 = 6.35 adds 6 attack over nq
+2 Crab bow:
127 attack x .1 = 12.7 (they round up) so it adds +13 attack over nq.
+3 Crab bow:
127 attack x .15 = 19.05 so youll have 19 more attack over nq.

No matter what kind of equipment it is, it has the same effect, be it a DoL tool, a DoH tool, armor, or a weapon. Any time there is 1, 2, or 3 stars next to a stat it is increased by 5%, 10%, or 15% respectively.

I need to test this more to confirm it, but i think this is 100% true on every item.