I dont use improvise, i get unstable 1/4 to 1/5 of my high level synths, so its not really worth it. This setup will net between 400 and 500 quality using NQ materials more than 95% of the time, its a very safe setup, but its very hard to go over 500, its not luck based like most of the setups posted here (which depend on unstable orbs or getting lucky with great sucess).

i start with standard, if i fail i use it again, then i use careful until i get a rainbow orb , then i pop Byregot's Blessing and perfection for a 90-130 quality, if i get red or rainbow for the next step i use culmination, if not i just use careful and culmination on the next orb. After this i use master´s mend (you can use whenever you want actually, but i like to save because it can change orb colors and probably stabilizing orbs) and keep using careful/standard, looking carefully to the progress and durability to get at least 69% and 20+ dura (if unstable) or 67% and 11 dura (if stable). Then i use another careful and piece by piece or by the book.

At least for me this is safest setup possible, sometimes if im feeling lucky i change perfection for purification and piece by piece for comfort zone. But this usually nets me more or less the same quality and feels more dangerous when i get unstable.