Everyone's been screaming about how to do this properly. So here's my idea for it.

When you synth an item/material you up the QUALITY. If you reach a certain quality you can activate the HQ/Rare meter. How about at the end of the synth you get a bonus bar of some sort that has a rapid meter that requires you to trigger within zone to get your rare or HQ synth?

There are two options in this method. The meter moves extremely fast and the higher QUALITY you got will make the meter move slower to trigger in the success zone. Or the other option is as QUALITY gets higher during the synth the meter target success zone increases.

Example: Cringes...Paper Mario shows this type of effort reward during battles with active skill triggers.