Something of a strange assignment iat!!
Taxying out - after a certain stage Autobrake MAX will be selected for the RTO case but it is not available until 72kts IRS GS.
Taxying in - the PF will have disconnected Autobrake in order to be able to taxi in - otherwise it will bring you to a halt on the RW!!
I am not aware of how you would calculate the decel rate from 30kts to a standstill with max manual braking in an A320. Experience tells me that it would be almost instantaneous as JT suggests. - an unpopular move!!
I would be interested to know the context of the task you have been set.
Cheers
Mcdhu