It's a very personal thing. I think, for example, the calorie counters on the ellipticals are very unaccurate (since the machines are relatively new and all the counters are statistic based). I also believe the most accurate are those of stationary bikes and threadmills.
I think the counter on the watch is VERY unaccurate, but that is only me. At best, it will be a ballpark figure, purely reference
For my own calculations I usually estimate 500 cals for each 45 mins @ 75-80% MHR... that might be off as well, since it depends on a number of factors that vary from person to person