I've seen previous versions of this question on the forum and the typical answer is to look at design information provided by diffuser manufacturers.
As others have replied back, the standard is to list throws at 50 fpm as a minimum.
Typical installations would have ceiling diffusers. I could reduce the throw to non-isothermal conditions (although IEQc7.1 doesn't specify supply conditions), assume that my air is horizontal until it hits a wall, then figure the additional distance down to 3' and see where that falls in the diffuser info. This would be if someone is next to the wall. If someone isn't next to the wall, there is no significant air speed. [Yes you could have two ceiling diffusers blowing at each other and resulting in airflow down.] At least nothing that can be calculated without serious computational flow dynamics.
Is everyone just pulling numbers out of the air (no pun intended, thought that I'd keep it safe for work) for this credit?
IF I could enter in the form "less than 40 fpm", I could probably make a good educated guess if that is true. Entering other numbers is (typically) completely bogus.
thanks