Suggestion for the Cut Down rune: from (your MHP) - (tgt's MHP) to (tgt's MHP) - (tgt's base HP)

Cut Down is a very inconsistent rune: it has anti-synergy with Triumph and Overheal, since both scale with your Health, a stat that decreases the efficiency of Cut Down; it also punishes people with high base Health like Kayle or Tryndamere, both champions who could use a anti-tank rune since it's part of their job to melt them; it also doesn't work good on solo laners as much as on ADC's, since they usually have less Health due to lower level than the rest of the team.

My suggestion is to change how the rune calculates the damage boost:

On Live Servers:

Deal 5% to 12% more damage to champions, based on how much more maximum health they have than you. Bonus damage scales up linearly against enemies with 10% to 100% more maximum health than you.

Suggested:

Deal x% to y% more damage to champions, based on how much bonus Health they have. Bonus damage scales up linearly against enemies that have 10% to 100% of their base Health as bonus Health.