[Sabaron]

HQ's go in tiers. Tier 0 is <11 levels above the cap and have an estimated hq rate of 1%.
Tier 1 is 11-30 levels above the cap and has an estimated hq rate of anywhere from 10-15% depending on who you talk to.
Tier 2 is 31-50 levels above cap and has an estimated hq rate of 25%
Tier 3 is 51+ and has an estimated hq rate of 50%.
Tier 1 is tricky as it seems to be different with different synths, but all in all you'll get around a 12.5% hq rate when strectched over a large enough base.

You'll also hear of people who get two hq's in a row on a tier 0 synth or people that go 150+ synths and dont get any hq's. There is a certain element of luck involved, but probably if you parsed every crafter in the game it would still probably even out to the above percentages.

I've done quite a bit of alchemy and I don't get anywhere near 10% HQ @ Tier 1. I think Tier 1 is more like 5% and Tier 0 is actually a fraction of a percent--I've gotten maybe 3 Tier-0 HQs over the entire duration of my Alchemy, and in general, my HQ rates reflect this:

Tier -1: Anywhere from a 5% to a 50% break rate which increases dramatically on a per-point basis up to -10.
Tier 0 (+0 -> + 10): 5% fracture to even, few fractures roughly 1 unit of product per synthesis.
Tier 1 (+11 -> +20): low fracture rate depending on recipe (some recipes seem produce fewer fractures than others), 10% increase in product per synthesis.
Tier 2 (+21 -> +30): 20% increase in product per synthesis.
Tier 3 (+31 -> +40): 30%
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Tier 7 (+71 -> +80): 70% increase.

Tier 7 is where my current Beeswax production lies--I get roughly 70% more beeswax than NQ would be expected to produce (including breaks). I keep track of this loosely by simply counting the number of crystal-stacks I use vs. the amount of product. Since I'm only talking about overall volume, this includes HQ1, HQ2, and HQ3. Because of this fact, it does not reflect the actual number of High Quality Syntheses which is, in essence, lower than my PPS rate. Of course, with non-quality consumables (i.e. HQ's that produce more product rather than a better product), I could really care less about the number of HQ's as I'm more concerned with the overall amount. A Durable Goods merchant would have better insight into the actual rate of HQ.