My plan back in the day was to use a % of synergy to rank decks with a hypothetical perfect mana base (all land enters untapped and tap for half your deck colors) being worth 20% and the remaining 80% being calculated by the amount of your deck that contribute to your win condition and the efficiency of those spells.

This won't necessarily describe "power" in the sense we use now as much as the efficiency of success, cheaper spells, functional duplicates, and perfect mana well inherently be more likely to do what they do than the same deck with pet cards and filler.