Manually or computerize 1024bytes = 1 M.b (Mega bytes)
1024M.b. = 1 G.b (Giga bytes)
1024G.b = 1 T.b (Tera bytes)

And so On. now 400 G.b. = 400 * 1024 = 409600 M.b
If U R getting 350000 M.b = 342 G.b (approx)
of the free usable space.
Rest are used by the system to share with the system files.