The Commission notes that the current number of EU citizens is just below 500 million, and that accordingly, a 0.2% threshold would be the proportional number per Member State.

I basically would not have a huge problem with a requirement of 1/3rd of the Member States and 0.2% of the population per Member State, since at that level it would be practically difficult for a single Member State to dominate the list of signatories.

Alternatively, though, we can see that the largest current Member State, Germany, has about 1/6th of the population. So I could also see a requirement of 0.1% per Member State coupled to a requirement that no single Member State provides more than, say, 1/5th (20%) of the signatories required for the threshold.