I doubt you would need synergy 60 to attempt the synth. The way synergy works is that you take the Synergy Craft Cap - Your Synergy Skill, and every time you feed fewell itraises the impurity ratio by that percentage (minimum of 5%). So if you tried this synth with absolutely no Synergy skill, you'd get +60% impurities every time you fed fewell. The higher the impurity ratio, the more likely an explosion. Explosion = loss of furnace health and greater likelihood that the synergy will fail before you reach the required elemental balance.

However, you can get the aid of a skilled Synergist to do all the fewell feeding for you. Anyone at 55 or higher shouldn't have trouble with a 60 cap recipe. Note that to do this, you need to undertake the quest to unlock Synergy. You just form party with the synergist, trade the ingredients to initiate the synth, then he feeds fewell and whatever else needs to be done until synth is complete.

(I dunno how much of this you already knew, but even if you already knew all this it could be useful to others.)