Its all a question of balance. What size elastic, anticipated quarry (size
of fish), line strength etc etc. You wouldn't want your elastic just
creeping back inside the bush when fishing carp puddles, you need stiff
resistance from the strike. Not a rule of thumb but I have found:-
Too many missed bites then the elastic is too slack
Too many bumped off fish the elastic is too tight

Experiment as posted replies in this thread and you will soon learn to tweak
accordingly. Always start off on the slack side though (for obvious
reasons).