It's hard to say what a par score is when only one team has batted, but given the opposition/conditions, it's hard to see how a team batting first would be happy with anything under 425 on this deck. Good chance England would have reached that by the end of the day too after Lyon's last over.
Unless they're confident of a washout on day 3, it is just a really strange decision against the best team in the world.
Even if you think your chances of taking a wicket is higher at that point in the day, England should have considered the wickets they'd lost up until that point. With the exceptions of Crawley and Brook, pretty much every other wicket was a result of England's attacking mentality. Not to have a go at that, but surely that would have suggested the Aussies were in a good place to see out the day.