http://falkennest.org/armory/index-en.html
With the introduction of essences came a great degree of customization. With this however, also comes a great deal of calculations to maximize what pieces of gear you choose and what essences to slot in to it. Math is not everyone’s forte and the choices can become muddied.
Ex. Runekeeper
You want to max out your Tactical Mastery: should you choose to slot Will essences or Tactical Mastery?
Supreme Essence of Will = 149 Will + 374 Tactical Mastery
149 x 8 = 1,192 Tactical Mastery (1 Will = 8 Tactical Mastery )
374 T. Mastery already on it
Total = 1,566 Tactical Mastery
Supreme Essence of Tactical Mastery = 1,310 Tactical Mastery + 374 Critical Rating
Therefore, you should choose Will over T. Mastery as it will give you 256 more points in to Tactical Mastery.
Fortunately there is an online tool (available in English and German) to help you plan your gear and essences without relying on in game trial and error.
http://falkennest.org/armory/index-en.html