Don't wait for Ming to explode. Instead, go for development of at least 300, then decline tributes till Ming cancels, then ready yourself for them to declare on you. Or if they don't declare, wait for their Mandate to plunge before going after them.
Most people will advocate going fast, i.e., start unification of China before 1500. You'll need to weigh between a couple of issues.
1. Manchu or Qing. Manchu government allows your horde army ratio as well as the extra shock on flat terrain. Big advantages in early war. But Qing allows you to core Beijing instantly and provides greater stability.
2. Take the Mandate fast or wait. Taking the Mandate early allows you to start accruing Mandate points, to climb the Mandate reforms. Once you hit the RCC reform, it makes taking the rest of China lesser of a pain. However, once you take the Mandate, you're reliant on consistently making Ming your tributary, to accrue the Mandate points.