practicing HIM nutrition

I am doing the Racing for Recovery Half Ironman in 7.5 weeks (my first HIM).  My long bikes are now 40-65 miles so its time to start practicing nutrition.  I am trying to figure out how many calories I should be targeting during practice.   I am 200lbs (90kg), and based on my current training, estimate that the race will take me 6:10.  Here are my calculations:

Needed Calories:
   Swim: 12 C/min * 45min = 540C
   Bike: 15.3 C/min * 190min = 2900C
   Run:  15 C/min * 130min = 1950C
Total Calories needed = 5400C


The bike calories seem high but I have gotten similar results from different sources.  I am figuring that I can get 1000 of these calories from fat stores (~20%), and that glycogen stores will account for another 2000 calories.  That means I need to eat 2400 calories before/during the race.

Normal pre-race breakfast: 300C
Pre-swim gel:               90C
5 gels during the run:     450C

That leaves 1500C  to consume on the bike (about 500C per hour).  Does this seem reasonable?  Feel free to challenge any and all of the above assumptions.

Thanks.