All you have to do is look at the spec sheet:
http://www.fomausa.com/pdf/Fomapan_400.pdf
Foma uses Microphen to determine speed, and the real speed in Microphen tops out at 320, but only when the gamma is way higher than ISO standard. Foma calls it 400 speed because it’s within a third of a stop at its max in Microphen. That’s fine.
In the real world, using D76 and developing to a gamma closer to ISO standards (0.60), you should expect to shoot it at EI 160-200 according to their published charts. In Xtol, at 0.60 gamma, you get about a third of a stop more speed than D-76, and in Fomadon LQN you get about another third of a stop of speed over XTOL at 0.6 gamma. Foma’s spec sheets for their films are shockingly accurate, if you look at them and read the charts that they have in there for the developers that they’ve tested.