Alright, after clearing some of the numbers up, and talking with the delightful Zusterke on twitter, I'm reposting the numbers on Evangelism and Archangel, to give you an idea of mana pool sizes, and what you need to shoot for in order for Smites to not be a mana sink as a Disc priest.
Here is your hefty disclaimer, however: As I write this, a new beta build is being implemented, and since, of course, this is beta, the numbers are quite likely to change before anything goes live. These numbers are for informational purposes only, and I will likely revisit them once changes are live.
All that having been said, let's take another look. For the purposes of simplification, I am going to focus on the level 85 numbers, and then I will explain them.
I included the equation in this table so you could see how I determined how much mana you would need. As you can see, these numbers are much less alarming than I previously thought, and you do not really need to worry about if you will be able to get that much mana - I assure you, you will. At level 84, I've got over 50k mana from just quest rewards.
The "Total Mana Spent" column is a summation of the total amount of mana you've spent on Smites in order to get to that stack of Evangelism, with the mana reduction costs factored in. This table is meant to be read from left to right, and it may seem confusing, because it's a built process - You start with no stacks, you cast one smite that has no mana reduction, and you now have one stack of Evangelism. If you cast Archangel at that point, you will get 3% of your mana back, and then you will have no stacks of Evangelism, as you have "used them up".
Therefore, the first column is to be read as the number of stacks of Evangelism you have *before* you cast the Smite being calculated.
Please note that I rounded down for everything, and I did it at each step of the way, so the numbers are not exact. Zusterke ran a similar calculation, and there were small variations in our numbers for that reason. This is meant only to give you an idea of how to process works on your mana pool, so if you're a theorycrafter, don't take my numbers and try to do something clever with them. They are all clevered out, thanks.
PS: If you don't know who Zusterke is, get thee to the PlusHeal fora. Not only is he a mathematical whiz, he's also an incredibly nice guy, who is very helpful to those with questions.