My General Idea is that a comparable number of uses of an Eternal Spell should cost at least as much as 7 copies (ignoring the 6 of a kind rule) of an existing similar card or theoretical card.
Hmm. Out of curiosity, is this just hypothetical or actually drawn from a comparison with an ingame card such as Fractal/Mitosis? Could you clarify this a bit? (I'm not sure if you simply mean multiplying the card's cost by x7 (which would make it 21 O.o) or if 7 copies has a different cost value.)
Not yet. Playtesting a 3 |4 version would give data.
The current playtesting system requires at least two people to be able to playtest. Would anyone be willing to volunteer? (For more info, see the 'Card Idea Playtesting' link in my signature.)
The 7 copy mark is an attempt to generalize niche balancing. Most cards cannot break the 6 card limit. Making Eternal inefficient at less that 7 uses helps preserves existing niches.
Process:
Create an Eternal spell [Ex: Summon Photon]
Find in game analog to compare to. (If none exist then make a theoretical card to compare to.) [Photon]
Find the cost to use that analog 7 times. (Pretend the 6 card limit does not exist. I would say multiply 7 but that is not always the same thing.) [0 quanta + 7 cards]
Find the cost to use the eternal spell 7 times. [7Z quanta + 1 card]
Set both costs equal to each other. (This should look like "X quanta + 7 cards = Y quanta + 1 card") [0 quanta + 7 cards = 7Z quanta + 1 card]
Solve for the cost of the eternal spell. [6 cards = 7Z quanta, 6 cards / 7 = Z quanta, 12 quanta / 7 ~= 2 quanta]
If you PM me on Saturday or Sunday, I might be available to playtest.
However I recommend others also try playtesting.