take the ratio of the base #tiles to desire #tiles. For example, Jackal (very fast) requires 6.5 tiles to charge, and one wants it to be 6. Calaculation: 6.5 / 6.0 = 1.0833. So you need 109% mana boost to get to 6 tiles (rounding up).
The sequence for most heroes is slow to fast because 12/11 < 10/9 < 8/7 . Very fast breaks the pattern because it’s only 6.5 tiles, not 7, so we only need to trim off half a tile, not a full tile. It’s even easier to get Very Slow down to 13 tiles (from 13.5): 4%, which is the base value of a mana troop.
Remember that Lancelot and Khagan boost mana regen in the same manner as troops, so they work synergistically.