How does the equivalent exchange of alchemy work? Alchemy, in Full Metal Alchemist, is based on the concept of 'one needs to provide materials of equal value compared to the thing one want to create' (equivalent exchange).
But, how does it exactly work.
Can this amount of required materials be calculated by the alchemist? If so, how? Are there some sort of lookup-tables?
Or does one need to guess and provides something of more value to be on the safe side? If so, can one become a more skilled alchemist by experience of successful guesses?
Also, different alchemists might have different specialized skills. How does that work? For the guessing part, I can imagine that one have a preference for certain kinds of alchemy and thus gain experience with a specialized skill.
Edit: Madara Uchiha's answer suggests that there are always materials required. I was also wondering about the situation where this is not the case and the alchemist was successful none the less.
For example:

 In the end Edward trades his alchemy skill in exchange for returning Alphonse's body to him.
Alchemy has three stages: Understanding, Break down, Rebuild.

Different alchemists have different specializations because of the Understanding part, learning chemistry in general isn't easy, and digging into a specific subject (For instance, the study of the atmosphere and oxygen in Roy Mustang's case), is even harder, hence specialization.

Matter of "equal value" means of equal "type" (Mineral to mineral, organism to organism, you can't turn stone into a flower), and of equal Mass.

This works well in accordance to the physical laws of matter and energy conservation. You cannot make matter disappear, nor you can make it appear out of nothing. You can't input more matter to be on the safe side, that matter won't be used for the alchemic transmutation (at best), or result in a rebound (at worse).