Adding a bitmap, v 0.90 superblocks

I would like to add an internal bitmap to my RAID1 array.  It uses
v 0.90 superblocks.

Reading the man page, I believe I do this with:

"mdadm --grow --bitmap=internal /dev/mdX"

correct?

How do I know how much space is available on the superblock when mdadm
will perform it's computation "to make best use of available space"?
I'm curious because perhaps I should change to 1.x superblocks in the
future (for non-bootable arrays).

Regards,
Richard