Maybe I need to polish my Googling skills, but across ElitistJerks, IcyVeins, WoWhead and of course MMO-C I haven't been able to find any theorycrafting on this ability for MoP, and yet... everyone seems to agree that "you should stay at around 90% mana for max DPS". Where's the math behind this? What's the current formula? What's the DPS loss of mana capping?
While still old, a lot of the work done by Kavan and myself in this thread (elitistjerks.com/f75/t106141-arcane_cataclysm_updated_4_01_a/) is still relevant to Mana Adept today. Feel free to look it over and post any specific questions you may have.
As far as the question concerning the "90%" hover is concerned, the mathematical basis for that revolves around a few basic concepts: n.b. These concepts and the math behind them are (at the time of this writing) very much in flux mainly due to the fact that Arcane Charges are being heavily reworked on the PTR. I am of the opinion that, at least for now, we should look at these ideas as purely guidelines and not as hard fact, and instead, wait till the state of Arcane settles down a bit before crunching the numbers further and calculating optimal levels
1. "Gem Range" - Currently, mana gem returns on average 15% of your total mana. As a strategy, Arcanists try to remain within 'gem range' in order to be able to snap back up to 100% mana in order to maximize mana adepts bonus as required.
2. Cycle MPS usage - The second (admittedly more 'mathy') reason for the ~90% figure, revolves around the computation of cycle MPS (mana per second) usage. Naturally, this varies depending on which level 90 talent you use, but in the general form, the first few casts of AB in a new cycle will generally be mana positive (i.e. you will gain mana). Hovering around 90% mana will accommodate this fact and allow you to prevent 'mana overflow' and hence, prevent wasting mana. As any Arcanist might attest to, wasted mana is wasted DPS, especially given the tools Arcane has to turn mana into damage.
Basically, since the fist few casts of AB per cycle are mana positive, you need to maintain a sub 100% mana point in order to ensure you are not wasting mana.
A interesting side point to consider when pondering the nature of Mana Adept, is the fact that its benefit is linear. This is important to note even if only to realize that it is not something inherent to mana adept that makes us hover around 90%, but more the concept of maximizing mana regen usage.
Either way, there will be a significant recalculation that must be done now concerning optimal mana % points for each level 90 talent, especially now with the new (old?) 4 stack Arcane Charge. Basically what I am saying is, stay tuned! Math is incoming
Currently everyone is not saying "stay around 90%." The common practice is to stay as close to 100% without going UNDER 90%. Maximizing your # of AB's while above 90% is the key. Closer to 100% the better. Capping mana isn't bad, but remember this is when you should be mid cast of an AB, preferably shooting AM, AB right after.
My goal is usually to be hitting my mana cap right as my AB is going off.
I find this much more fluid in a Mastery build Arcane spec than Haste.
I know this isn't the numbers you were looking for but hopefully gives you insight on what people are saying and why they are doing it.