Leveling in P3 e/c

When using the leveling tool in P3 e/c the client I work for use the "preserve schedule early and late dates" option. If we are working to leveled dates and not the early start dates surely this should be unmarked to give the true total float. I have tried both methods within the schedule and both results are very different. My question is should this be marked or unmarked ? I am trying to find out which is the correct method. I believe it should be unmarked to give the total float based on the new leveled dates. Any replies would be grateful

Cheers