How much true damage conversion does Infinity Edge need?

With the upcoming Marksmen changes (I can't believe it, it is like a mini Marksmen Update), Infinity Edge will have its crit damage increase replaced by a Conqueror-esque true damage conversion on crit.

Now, how much damage should be converted into true damage?

Imo, the breakeven point should be at 150 - which is about equal damage to a single big Armor item (60+ Armor with around 90 base @18) enemy.

What amount of true damage conversion equals a 25% damage increase on a 150 Armor target?

1.25 DMG x (1 - 150/250) = TDC x DMG + (1 - TDC) x DMG x (1 - 150/250)

1.25 x 0.4 = TDC + (1 - TDC) x 0.4

0.5 = TDC + 0.4 - 0.4 x TDC

0.1 = 0.6 TDC

TDC = 1/6 = 16.7%

So, a true damage conversion of 16.7% would mean that a crit deals equal damage to a target with 150 Armor between the reworked IE and live IE.

Do you think a breakeven point of 150 Armor is realistic?