While I understand the reasoning, having both the base damage and the scaling scale with level makes the damage of the skill unpredictable. Almost all other skills are either "Does X bonus damage plus ?/?/?/?/?% of his AD" or "Does ?/?/?/?/? plus % of his AD"... generally it's easy to tell where a champion is investing points.

actually, if i'm not mistaken trundle's q and sejuani's W have scaling scaling and base damage.

Pantheon's old Heartseeker was like this. A flat bonus amount per rank + a % of total attack damage. Some champions still have this type of scaling in their abilities such as Urgot on Q and Trundle on Q.

The intention here was to allow 'tanky' Garens to still deal some damage since it now scales off total AD as opposed to forcing them to invest in AD.

Then couldn't it just be something like 80% AD all game, and just scale the base damage to be more appropriate?

Or, they could just make it 100 / 110 / 120 / 130 / 140% of his attack damage per second (no base damage). Give that the damage occurs every half second and can critically strike, be basically gets AOE auto-attacks for 2.5 seconds.

I don't think you understand.

If you put it at 80% of his total AD all game, he'll be doing less damage and you mind as well be auto-attacking. It is 70/80/90/100/110% so that it doesn't just wreck you early game and so that it doesn't do **** damage late game.

And as I previously stated, having it scale off of total Attack Damage means that 'tank' Garen players won't do **** damage (or not as **** as before).