Tempo = Board Advantage / Mana
If Tempo is going to be useful for anything other than being able to say, "That card really screws up your tempo" or "I had really good tempo that game", then it needs to be better defined. I think the formula above is a good place to start.
For our purposes, the goal in Magic is to reduce your opponent's life to 0. Toward that end, there are two main archetypes: Aggro and Control. (Mill decks are control decks. Combo decks work by trying to play a different game.)
For aggro decks, improving board position is the way to reduce the opponent's life to 0 as fast as possible. Control decks want to disrupt the opponent's board position, preventing their own demise. This leads to Zvi Mowshowitz's idea of the 'fundamental turn'- for aggro, the turn they win; for control, the turn it becomes extremely unlikely the other player will win.
Creatures are the most important cards in aggro decks, and for them the "advantage" attribute, I think, can be calculated. Power would seem at first to be the most important attribute, but creatures are not always in a deck to attack. Wall of Roots's role is mana acceleration and neutralizing the opponent's creatures, for example (abilities which are synergistic in control decks, which want as much mana as possible, and need to survive an aggro rush).
So let's start with a simple example: Eager Cadet. This card sucks. It's a vanilla 1/1 for W and a card. How much better is a player's board position when they have this, compared to not having this (all other things the same)? It seems very much like a "lowest common denominator" card, so let's assign it a value of 1. The board position value of all other cards can be calculated in terms of how many Eager Cadets they're worth. Because it's funny, I propose the unit be shortened to "Cadets" or abbreviated as "C" (eventually we might be able to say, "Yeah, that card's worth about 1.2 kilocadets (KC)" ).
This doesn't necessarily mean that a 10/10 creature is worth 1 decacadet (DC). It can only block one creature; 10 Eager Cadets can block 10 creatures. If it attacks, it can be blocked by one creature; 10 Eager Cadets require 10 blockers to be completely stopped. On the other hand, Tempo does not care about life total (although it's certainly possible that it cares about change in life total- If you're dropping 8 life per turn, you're probably getting out-tempo'd pretty hard).
This is getting long so I'll stop here.
