That’s what FS means. FS = Fair Share. This amount of damage will guarantee one top tier loot item. For the raid achievements on the NM raids, you have to do at least 0.5 * FS.
FS is calculated as Starting HP of Raid / Maximum number of participants in the raid.
OS is Optimal Share, and it is the amount of damage that gets you the most loot. This is FS * a multiplier based on the size of the raid.
There are a couple of raids this doesn’t apply to – World Raids, Space Pox and Quiskarian Temples.
The best way to find out the FS/OS for each raid is to get one of the scripts from Doomcat or SReject (search the full forum, should be near the top)