I always have credits left over but my flying is half international and half domestic in M class Flex passes where I can fly a lot and use substantially less points for upgrades compared with regular non-M class Flex fares. I also buy F/J on UA and CA flights.

Also there is lumpiness to my flying so I am usually flying half my miles in the last 3 months of the year which gives me some credits that may last more than a year (depending when the cutoff date is).

I used to get eupgrades for flight pass purchases but because I had leftovers so many years I went for 2500 status miles for each flight pass purchase since my flying has slowed down a bit and I am now usually in the 85-90,000 Q miles from actual flying. With 4-6 flight passes a year, it works out fine and hope to have fewer/no eupgrades left this year.