Its not a "hit this number for quality and you will HQ", its more "the higher your quality the higher your chance for a HQ".
I've seen a synth hit over 300 quality, so just getting to 100-150 probably isn't that high a chance of a HQ.
Also, "Touch Up" at the end of a gear/weapon/tool synth appears to give you another "roll" at a HQ result based on your quality at the cost of some durability.
If you have enough durability left, you can "Touch Up" multiple times, but you need to find the balance between using Bold more often to get a higher quality score (and having less durability remaining at the end), and leaving yourself more durability (but having a lower quality score).
XI Wiki


