But it was Boba Fett who alerted the Empire that they had gone to Bespin! The fleet had jumped into hyperspace and gone off to "every possible destination along their last known trajectory", as ordered by Mr Vader.

I like to think that Bespin lay in another direction and that it was only due to Boba Fett's presence of mind that the Empire had even a chance of catching them. Fett probably contacted the fleet once he was fairly sure of where they were making for - Fett has a history with Han Solo and perhaps, therefore, knew about Lando. He put 2 & 2 together, called the fleet, they arrived ahead (due to their greater speed) and prepared the trap.

BTW, who was it that decided how far away Bespin is from the asteroid field? Maybe they overestimated?