I got mastery 7 on rengo the hard way, believe me i did the maths twice for this argument. Rengar R jump is just a regular crit, which means doubled AA damage. earlygame and midgame that will (unless he goes 12/0 or worse before 15 minutes and buys items asf) never be above 200 ad, assuming he got all 5 bonetooths.
200 ad x2 equals 400.
standart lvl 9-11 armor for an adc around that time will be somewhere around 40 to 50, which equals an 20-23% raw damage reduction for physical damage.
400 - 20% is 320
Duskblade around lvl 9 does around 150 damage +/- 20 if i remember correctlym meaning you are at 470.
NOW, add ninja tabi.
470 - 10% equals, rounded down, 420 damage, now have that extra armor counted in and you are at around 400 damage.
add lethality to negate some of the armor, for lvl 9, and you are more or less at around 420 again.
so...how is 420 anyhow close to a minimum of 700?
And as a mage you can be relevant without morellos. Go Mana flow keystone and stop spamming your skills to clear waves. Mana management should be a Thing.