Caution: There aren't really rules for Artifact or Charm design in EX3 right now, so everything you read below is based on my practice for homebrewing them. I also expect that the upcoming supplement Arms of the Chosen will flesh out this system much more, and may invalidate what I've laid out here.
Let's see at what Artifacts published in EX3 actually look like. When counting, I generally represent Innate Powers as though they were Evocations, but the Exalt gets them without spending experience.
Rating Innate Powers Evocations Capstone Essence
Beloved Adorei ●●● 2 5 3
Brilliant Sentinel ●●● 1 3 3
Moonlit Huntress ●●● 1 1 2
Shining Ice Mirror ●●● 1 4 3
Spring Razor ●●● 1 4 3
Freedom's Cadence ●●●● 1 5 3
Hunting Hawk ●●●● 1 5 3
Dauntless ●●●●● 3 4 3
Black Wind ●●●●● 1 5 4
Volcano Cutter ●●●●● 2 4 4
More Artifact dots, more Evocations? Not really.
As you've noted above, the only real guidance that is provided in EX3 is that bigger artifacts have more Evocations. Unfortunately, this isn't really supported by the example Artifacts at all. There is somewhat of a trend that higher-rating Artifacts have more powerful Evocations, but it's not very explicit.
Evocations are just Charms
When I design Evocations, I don't look at the rating of the Artifact to supplement their power level. Instead, I try to build Evocations that fit in with the general cost/benefit of ability charms at the same Essence requirement, and price them similarly.
Disclosure: The finer points of my Charm pricing strategy are from a document called "Balancing Magic in Exalted" that one of my players shared with me, but I can't find the original source for. I've kept to general advice here, but that article goes into an incredible amount of detail.
The Artifact rating should determine two things:
- How powerful the Innate Powers of the Artifact are. I usually write innate powers as charms, but the character gets them for free by attuning to the Artifact. Most Innate Powers are equivalent to Essence 1 charms, but sometimes they're more powerful, and those should go with higher-rating artifacts. Also consider that multiple permanent effects should increase the rating of the Artifact.
- Higher capstone charms. Essence 4 Evocations probably mean that you have Artifact 5. Essence 5+ means you have Artifact N/A. These are seriously mighty powers, and should have an appropriately-sized requirement tree.
As with all homebrew, your Storyteller has full and final discretion over whether to allow the Artifact, its Innate Powers, or its Evocations into the game.
But how do I get the Evocations into the Artifact?
Unfortunately, the only charm in EX3 that dealt with adding evocations, Design Beyond Limit, was based on earlier rules and removed in a later revision of the PDF. I present a custom charm replacement here:
Design Beyond Limit
Cost: Variable; Mins: Craft 4, Essence 2
Duration: One Extended Action
Prerequisite Charms: Durability-Enhancing Technique
The Solar Exalted are not bound to follow the present nature of the world, but instead raise it along the path to ever greater glories.
The Exalt chooses a number of new Evocations to infuse into the Artifact. This Charm creates an extended reforging action with difficulty 5, no terminus, and a goal number equal to (number of Evocations x 10). The interval for this extended action is (10 - Essence) hours. For the entire duration of the extended action, the Exalt must commit one willpower, plus an additional five motes for each Evocation beyond the first she will infuse into the Artifact. At the successful conclusion of the reforging action, the owner of the Artifact must pay one experience point per Evocation. The new Evocations are then infused into the Artifact, but lie dormant and must be unlocked by its owner through use of the Artifact.
Special Activation Rules: To use this Charm, the Exalt must have an undamaged Artifact to reforge in her workshop, and its owner must have unlocked all of the Artifact's existing Evocations. The Exalt must also be able to satisfy the Essence prerequisite for any Evocation she infuses into an Artifact with this Charm, however she does not need to have purchased the Evocations herself.