They are viable by level 7 which is only a level off. I did not say your idea was bad. I am talking about a greater flaw with these specific runes. I feel that at their max potential they are still to weak cause of the level curve and the way the game works.
I think you found something bigger here than you are seeing. AD runes currently function best at getting to level 6 and your first big item. This why they are efficient. At late game the +-10 damage does not mean as much as it does early game. 10 damage means more when your health bar is 500 compared to 1000.
These scaling runes will never be efficient with current scaling. They are a noob trap in the system. Most of the time runes are meant for early game advantage. Very few runes scaling runes are optimal, the excepts being MR,health and AR per lvls.
The early advantages matter 90% of the time the most. Our base stats are so bad that we are fragile or weak (depending on the class) with out these bonuses. What is the point of scaling AP blus when you are getting out damages and can not trade and CS?
Like i said before your sujestion really does not matter. I feel like it is a band aid on a bigger deeper issue that you have pointed out.
For the record per red flat ad versus scaling ad its aprox 1.5 at level 18. times 8 (cause of luck crit rune) it is only 12 more AD late game.
