Subaru Legacy L Wagon coolant loss issue

1. 6/1/2005, 76,255 miles: Car was purchased.
2. 11/12/2006, 90,308 miles: Coolant replaced without coolant conditioner
3. 11/30/2006, 90,820: Coolant conditioner added improperly to expansion tank, coolant added to expansion tank to bring up to Full level (It's unclear what level it was at when this was done.)
4. 2/24/2007, 93772: Coolant added to expansion tank to bring from Low --> Full
5. 2/25/2007, 93,784: Coolant conditioner properly added by draining 125 ml of coolant from radiator
6. 2/28/2007, 93,849: Coolant down 1" from max level on expansion tank
7. 5/28/2007, 96,767: Radiator pressure test, 23 psi, no signs of leakage
8. 7/1/2007, 98,403: Coolant added to expansion tank to bring from Low --> Full

So, the only truly accurate estimate is that the expansion tank needs to be refilled approximately every 5,000 miles: 98403 - 93772 = 4631 miles. The first refill after 2952 miles was after the coolant conditioner was improperly added to the expansion tank. Next November, should replace coolant again and add coolant conditioner *first* before putting in new Subaru coolant. Maybe take off top radiator hose and add from there to help conditioner get to the heads.