New Prius

Hey guys, I just bought a silver 2010 Prius III with nav on Saturday and couldn't be happier with it. After my first weekend of driving I already have about 53-54 mpg and having FUN doing it. I didn't expect it to be so fun to drive, especially in power mode. Now here are my questions

1) How do you do the mpg calculations per fill up if you're filling up at 1/4 of tank or other random points every time.

2) What mode is it in when you're not in any of the ev, eco, or power modes and is there any reason to be in this mode, since the other 3 pretty much cover everything?

Thanks