What is the ideal percentage of CO2 in the atmosphere?

Carbon dioxide is an essential vitamin for plant growth. More is better because plants grow faster with more available carbon dioxide.

Ignore the climatologists. They have no testable hypothesis, only a religious conviction, and a lot of falsified data.

There simply isn't one.... if you ask the question that generally.

Plants generally do better with higher atmospheric CO2, up to a point. But CO2 concentrations effect average temperature.... so there is a point of diminishing return for the plant. And that point depends upon the plant.

Most animals do not do better with more CO2. It is a poison to animals that breathe oxygen, and like plants they are generally adapted to a particular temperature range. Getting outside of that range is not good for them, nor their long term survival prospects.

@Ozy: "The Earth is warming" and " The primary cause of planetary warming is greenhouse gases" are BOTH most certainly testable hypotheses.

Were you dropped on your head as a young child?

0.4 %.... not sure.....