Flash, as I recall, is equivalent to 600 potency (it's been a long time since I tested it, but basically it generates equivalent enmity to an Overpower or DRK thing). In other words, it's as if you've used a 600 potency attack that scales with attack power and determination. However, it's a little misleading to just leave it at that, because
1 - Flash does not count as physical, so it doesn't benefit from Fight or Flight
2 - Flash does not benefit (iirc) from Crit
3 - Flash does not have a "damage range" of +/- 5% like most attacks do
4 - Flash does no damage, so the total potential enmity generation of your party members is higher than it would be if you were a DRK or WAR. There's more HP left over for party members to hit, so their maximum possible enmity is higher.