What you're looking for is something like TreeCalcs over at ElitistJerks. This addon is only and will only be used to track mastery values added, not overall healing gains from things besides mastery as well.

TreeCalcs is an amazing utility for resto druids:
http://elitistjerks.com/f73/t116144-resto_treecalcs/

Correct, your combat log is the results. How this works is there is an event that your combat log spits out every time it's updated with any line. We read those events, looking for specific ones (basically when heals and auras (buffs) are applied to players). From there, we can look at the event and determine if the cast owner is the player (you). If it is, we can then capture more information on that spell. How much it healed for, how much it overhealed for, how much mana it cost, etc.

So what we do with that information is save it into what are called Lua tables.

Now, the tricky part is actually determining if mastery was applied to the heal. There is currently nothing in the WoW API that just returns true or false, so we have to do it manually. So what we do there is use a seperate Lua Table to track when hots are on players.

Let's say you cast Rejuv on a Player X. It would create a spell entry for Player X in the Rejuv table that says "ture" (for yes, there's a hot on the player). Once the combat log tells the addon that Rejuv has faded, it goes in and says "false".

What that allows us to do is every time a heal is done by the player (you), we check the table that contains all the active hots. If one is active on the player we just healed, we know mastery was applied and handle it accordingly.

Unfortunately, because the way mastery works, I do not believe there is any other log system out there to compare this data to. I do know it's being tested by many over at elitistjerks.com though.

Lastly, I would say this addon has 99.9% accuracy. There are times when the combat log (due to latency) tells your client that the Rejuv wore off before the last tick of the heal. Because of that, the addon will not apply mastery to the last tick of Rejuv for that case. This IS NOT occurring for all spells cast, and happens very occasionally. Given the overall base size of the hots at the end of the recording period, the affect on the data will be minimal.

Lastly, the data currently tracks only while you're in combat. You can disable this using the options if you wanted. All that info can be found on the main page.

This was probably more info than you wanted, but enjoy.

This will probably be in future releases. As of now, I'm not sure how useful this addon will be in say 3 months when we start to understand exactly where mastery fits in our overall healing. I think we're all starting to get a good feel of how good it is now.

That's not to say I wont' support or improve this addon in time, but I'm not actively working on it currently. The next release probably wont' happen until a few weeks after the patch.

But I can certainly look into adding some saved variables in and an option to reset the data on zone in.

I added in two new slash commands in the latest version.

Can now track effective mastery versus effective healing as well as just for crits.

I'm still seeing errors when the SPELL_AURA_REMOVED fires BEFORE SPELL_PERIODIC_HEAL for some reason. In reality they should never fire out of order, but after talking to some people, I guess it's quite common.

Probably going to stop removing players/spells from the tables, which will increase their size. The table would only exist on the session and doing /dm reset would clear the table so I'm not too worried about it.

Still need to test it out and see if it stops the errors I'm occasionally getting.

Alright, so we'll have one value for "effective mastery healing" versus "total effective healing" [including crits] and we'll have another data table that does the same thing, but just for crits? That should be pretty easy.

I've got the code written for the first one, just going to work on the crit one now and then test it. Should have this addon updated in a few hours.

Ok, I'm with you now. I wasn't thinking about dividing by 1.10 (with 10%) to get back to the original heal.

Summing that should be pretty easy. What about the total value we should compare to? What heal value should I be summing there? Total heal including over heal, just the total mastery including overheal, just the total actual healing done excluding overheal?

I see what your formula is doing, but how is it useful when we compare it against X (where X is the above choices)?

EDIT:
Thinking about it more, we'd want to use the actual heals without overhealing right? So then we could say my mastery accounted for X of Y of my total effective healing. Right?