It would seem that Broadway Spotted keeps track of rush, sro and lotto giving you a sense of how early you need to go.
Do a google search on Broadway Spotted Rush and you should find it. Looks like they update it once a week.
I can't vouch for its accuracy, but it looks like it would be helpful.
Or just go here. http://www.broadwayspotted.com/category/rush-report/
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