The idea is, that at few dots, the necromancer gets a strong bonus to that or those few dots, and at more dots, the bonus weakens.
T = Tome rank {1, 2, 3, 4}
D = Number of dots on target{1, 2, 3, 4, 5}
Damage bonus to all dots on target = (T/4) / (D/1). Or (T/D)/4
To make it from a damage...