Unless the race was a timetrail, that won't work unfortunately. The calculator assumes no drafting, so in the pack your average wattage would be lower than its numbers. On the other hand, if this race was fairly spurty, like a crit, then average wattage isn't a good measure of your ability anyway. Your body pays a penalty for the inconsistent pace, so the more variable the pace, the lower average wattage you can achieve. That's why "normalized power" was invented, but that's a whole 'nuther topic.