The first line should be something like "...: X cylinders, ..." and next line should say "... cylinders of Y bytes, ...". (X-1) * Y is the total usable size.

Z = 1073741824 ; number of byes in your linux partition (1gig)

A = (X-1) - Z/Y
B = A - 1

and then run
That is how you determine the partition sizes.

As for not being able to unmount mmc1, you need to stop whatever is accessing it before you can partition the card. If the "pwd" command returns /media/mmc1/* then you need to change directory to get off the card.