Is There Any Logic to Synthesis?

[Estellios]

Raldo's information is correct and generally the rules I go by while crafting.

If you are working on a synth above your level for XP purposes, it's best to wait if the orb is Red or Rainbow because a failed action is wasted Durability. I usually wait 1-3 times, but don't wait too many times as the cost of waiting goes up each successive time you do it. If you ever end up getting abilities that guarantee success on your next action, though, then it's best to use them on Red or Rainbow =P

Also, try to focus on Standard while you get used to it, and throw out Careful Synthesis as you near completion and your Durability is good. Increasing your synth quality gives you huge XP boosts, but it takes practice to learn your limits.

Rapid synth should ONLY be used with a Guaranteed-Success ability or if you are almost out of durability and have no other way to finish the synth - successful rapids don't use any durability if they succeed but will hurt a ton if they fail.

I'm linking the lodestone guide again, just in case you missed it in Raldo's post. The lodestone guides have a good deal of information and they may be able to answer some questions you may have on other parts of the game.