I get you :-(

With independent jaws, there is really no easy way to center the jaws
(that's why the self centering is called that way).

I'm not an expert but it seems that measuring the spaces between the
jaws (with something accurate) will give you an approximate estimate
(if the distances are the same between all jaws - then they are each at
the same distance from the center i.e. centered)

I'll say it again, I'm far from the expert and I hope I'm not
misleading you.

You should know that there are some neat uses for your type of chuck,
such as mounting irregular pieces and such.

All the best
Moshe