Help booking Award Flight PHL/CHI/SEA/ANC

I'm trying to plane a vacation next year (June 2014) with the following itinerary:

PHL to CHI
Amtrak From CHI to Seattle
SEA to ANC
ANC to PHL

Theoretically, what is the least amount of miles for this trip? I'm not using United miles on Amtrak, but I want to for the rest of the segments. I did a quick search and found the whole segment for 42K, but I was able to get down to 35K if I split out the booking (not sure why)

I was wondering if there's anyway to get down to 25K. This is my first award booking, so I was trying to understand the 'open jaw' policy and how to best utilize the fare system for the highest reward. The same fare was $908 if purchased.

I have 50K miles and want to book this flight as soon as I can (unless there's an advantage to waiting)

Thanks!