Easiest thing is to call them and speak to a customer service rep. Just say "I need to know how far off I am from my next miles boost".

In my experience, what the customer service rep looks at will be more up to date than your online statement.

I'm a reserve holder and I'm really close to my second speend boost. It's coming down to the wire. I'm pretty sure AMEX's customer encounter records for me look something like this:

- 11/21 - called to check on spend for miles boost
- 11/30 - called to check on spend for miles boost
- 12/6 - called to check on spend for miles boost