My rule is +/-20% for both personal and business flights. It also depends on the routing, distance of flight, purpose of trip. I may come out of pocket a little more if it's from SFO-JFK vs. SFO-LAX or if the choice is coach on an airline I do not use much or FC on my favorite airline.

The other thing that I do to combat this is to get comped status on other airlines. For example, I sent a copy of my UA 100k card to Delta and got comped their 50k Gold level - even though I had never taken a DL flight in my life. However in February I need to travel ATL-LAX and they had the best non-stop as well as the best fare by over $200, so I flew DL and used 3 of the 4 upgrade points they had deposited into my account. I've done the same on US Air as well.

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Ken in Sacramento