Actually what 400 at 16 minutes means...

You have a sensitometer exposure of -0.2 log mcs (or 0.630 mcs)

What you do with this information is up to you.

I basically mark it as 400 on my graph and then any other film I test, I read the speed right above where the curve crosses 0.1 density.

So expired films, experimental development techniques, anything you do... In one test you will know the EI to shoot _that_ film at ... when you develop it _that_ way.