Renascence

What is the best level to renascence your Heroes?

As soon as it is possible...just make sure your assembly is 9 levels higher than heroes level to get max benefits.

from personal experience, and testing, it dosnt matter how much higher your assembly is compared to your hero, since i renned, heracles, siren, when their respective levels was equal to the assembly levels and i allways gained max levels

Just to clear this up a little bit. The difference of 9 levels should be between your assembly level and the hero's Renascence Pre-reqs level, not the actual level of the hero himself.

If your assembly is level 80 and your hero's pre-req is level 67 then you can ren anytime after level 67, obviously you can't ren before that. But if your assembly is level 80 and your hero's pre-req is level 76, you should raise assembly to at least 85 prior to renning that hero, in order to maximize the benefit of renning.

In the first example, it is best to ren right at level 67 though so that you don't waste time training beyond that. The only time I might wait to ren was if I was trying to get a hero into a new piece of equipment before renning.