Except if you wait until you're 75% of the way done and you use this, it won't complete the synth for you.. :X It's based off of remaining progress, not total progress.
(If a recipe has 100 progress, and you use this at the beginning (0/100), it will give you 33 progress. If you have 90 progress (90/100), and you use this, it'll only give you 3.. )