Need Help! Edit tool lenght by PGM

Hi there,

I Am not a Fanuc programmeur, so I is probably an easy question!!
I have a vertical lath with milling options (rotating tools) with 18i-T control.
I am using a ballnosemill. In the tooltable is the length till the tip of the tool and the correct diameter/radius.
I want to load the tool and use is with the tool length minus the tool radius!!
So using the center of the ball!!
So I think what needs to be done is:
1: read out the tool radius into a parameter.
2: load the tool length minus the toolradius into its current memory.
3: start milling with the shorter tool (center ballnose)

How can this be done in the program??

Thanks in advance