My thoughts on your questions:

1. It doesn't matter how many miles you have in your account on any airline that I know of. It's all dependent upon how many miles you've earned that year or in your lifetime on some airlines.

2. I have a rough value for miles (between 1 and 2 cents per mile, depending upon the airline). If I can get a ticket cheaper than that, I'll buy it; otherwise I'll use the miles, no matter when/where I'm going.

3. As to whether to use the miles for flights or for upgrades, it all depends upon your wants and needs. If you're fine sitting in Y for your trip, why burn the extra miles for the up-front seats? Personally, if the flight is less than ~11 hours, I'll sit in Y. Otherwise, it's worth the extra miles to pony up to F.