Robert, for what it's worth, according to Dixondale, onions need 200 heat units to put on a leaf and every leaf makes an onion ring. The more leaves you have before the onion starts to bulb, the bigger the onion will be.
Here's a couple of videos:
https://www.youtube.com/watch?v=oALQHr15d2Q
https://www.youtube.com/watch?v=_g1-dAMjs0k