The challenge of synergy is to keep yourself awake while you skillup. Here is my short guide to synergy. Get ready:
1) Find a synergy recipe that is about 10 levels higher than your current level, preferably one that uses earth felwel.
2) cast haste on yourself
3) attempt to make that recipe using earth felwel until the duration of the furnace is about 50% - each time you feed a felwel the impurity will rise by 5 +
level difference between you and the recipe.
4) once the furnace is less than 50% feed it other felwel. If you get bored then try to actually make the item. I found that skill-ups are slightly more common as I approached completing the item.
5) repeat this process until you have hit CAP on that recipe. Then, find a new recipe that is about 10 levels higher than you.
There is no 'sweet spot' on getting skillups though so just push through. The reason I suggest a recipe that is 10 levels higher than your current level is that there is a trade off. If the recipe is too high, you will not be able to feed many felwel for chances to skillup per recipe synth. If you pick a recipe that is less than 10 levels you hafto worry about obtaining new materials for different recipes too often. The key to synergy is hammering out the felwel feeds, the more felwel you feed per hour for example, the more skillups you are going to get.
If you are trying to use a lvl 40 recipe while you are skill level 1. You will not be able to feed many felwell per hour. You may be able to get more 0.3 skillups , which is better than 0.1 skillups but that only lasts till bout lvl 50. So you hafto play around and decide what you like.