Total Float - Is it possible to change the driver to within a WBS Code?

I have a schedule, with 1700 WBS Codes.
Within each WBS Code I have 5-20 activities.
I would like the total float to be driven from a certain activity within each WBS Code. So basically every WBS acts as it's own entity.
Right now, all Total Float is driven from the Latest Finish Date of the entire schedule.

Is this possible?
If yes, please give me a hint :)