First Calculate the critical path

then calculate the longest path containing the activity for which float needs to be measured

and then just follow this: float= (critical path value/duration - longest path value/duration).

BK 

conceptually you are  correct.

for clarity

I want permission to make a little change, 

First Calculate the critical path

Then calculate the longest path other than critical path (second longest)  for calculating minimum possible Float (Total Float).

Then just follow this: 

Float (Total Float)= [Total Duration of project (at critical path)  - Duration at longest path other than critical path].