I agree with Peregrine, for pretty much any recipe ranked Mid-40's you're going to see unstables, and commonly 2 of them in the course of the synth. It's not an issue, because the game affords the necessary abilities to either deal with or soldier through unstables.

You're saying 10% or 20% failure is unacceptable, okay well you can with some work get that down to 3-5% on comparably difficult syntheses. It just takes a bit of dedication in getting the ranks and marks needed to qualify for and purchase the right abilities. Yes, there are abilities that actually have a significant impact on your synthesis success rate, and no they're not freebies given by just ranking up, and no, not every class gets them.

For one, there's Hand of the Gods which you can qualify for after doing the r36 ALC quest. It is not some sort of panacea, but it definitely improve your success rate. It affords a guaranteed 0 durability loss for 4 rounds on finished goods (i.e. Half-masks), and 3 rounds on materials. The sole exception is on chaotics that happen while under HotG, these will still reduce your durability, but! at a minuscule ammount when compared to a standard chaotic (I'm talking like 4-7 durability which is a joke).

Secondly if you know you're due for a particular kind of unstable for the recipe in question, you can prepare before hand by having the proper elemental brand. How do you know? Well if you've made the recipe a bunch of times before you know, trust me. For example, on Brown Toadskin Vamps (which I SPed on from r40-47 TAN) the most common unstables were Ice and to a lesser degree Wind. So every time I went to grind TAN I'd equip Brand of Ice and Brand of Wind. You use em reactively, they have a proactive use, sure, but you can't predict when an unstable will happen so it's useless to use them that way. By reactively I mean that when you're in the middle of the unstable you use the Brand and then you 'Wait'. I have yet to not have an unstable resolve by the 3rd 'Wait' when using the correct Brand, and more often then not the 1st or 2nd 'Wait' takes care of it.

Those are the two primary things I use for grinding out a craft through the 40's. If you take the time to acquire them, you'll be able to get the sort of success rate you seem to desire.

And no, I'm not missing your point. I understand it, you just want the failure rate to be lower across the board. Well I disagree, 10-20% seems fine to me, and for the people who take the time to acquire the right set of tools a lower failure rate is definitely achievable.