I have done

showfsp -d 15

This gives me report for no of cylinders which could free up when packed to desired FSP of 15%. Now the showfsp report has the following columns

databasename | tablename | fsp% | recoverable cylinders | current cylinder

Now the question is there is specific ration between recoverable cylinders and current cylinder which tells me that we should run packdisk

i.e sum(recoverable cylinders)/sum(current cylinder) if greater than XX.XX value then run packdisk.

Please let know in case of any clarifications.

Thanks in advance