Riot said that it has this formula in the patch 4.20 notes:
"Level 1: Immediately gain 0% of your growth statistic. All of your 'base stats' will be calculated off of this
Level 2: Gain +72% of your growth statistic
Level 3: Gain +75.5% of your growth statistic
Level 4: Gain +79% of your growth statistic
... Gain +((Level*3.5 - 7) + 72)% of your growth statistic
Level 18: Gain +128% of your growth statistic"
"At level 18, you've gained +1700% of your growth statistic (this is slightly more than the old formula)"
The key thing that whoever wrote that entry for the Wiki missed is that all champions had their base stats INCREASED, so I don't think they actually got it right. Ahri's base attack damage should actually be a little higher at level one than it was before patch 4.20, and then gain a more gradual amount of AD per level at early levels due to the curved scaling. This is basically so that champions don't get nearly as huge a power spike when they hit level 2 first, since the stat gain early is comparably small, while late game the gains increase. Also important to note, it should actually be EASIER for mages to farm with auto attacks since minions no longer scale up in MR and armor, only hp.