The Clan of Dragon

Reference: http://www.lordswm.com/help.php?section=32
Casters' spells
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Casters can use some magic characters do. Their spell power depends on the stack numbers.
- For duration spells, the spell is casted for as many turns as there are creatures in the casting stack.
- For damage spells, the formula is not linear, but it is unrevealed.

Troops Damage
1 20
2 32
3 42
4 51
5 60
6 68[?]
7 76[?]
8 84[?]
9 91[?]
10 97[?]
11 103[?]
12 109
...

^Estimated damage, as you can see each extra magi gets less and less.

1 20
2 32
3 42
4 51
5 60
6 68
7 76
8 83
9 90
10 96
11 103
12 109

very close to what you proposed