There is no adjustment on the deck belt.
Depending upon who made your mower for Craftsman it will be set by a spring on the end of the cable or by a spring on the deck.
Don't know which because 14,5/42 is the engine & deck size not the model number which is on the tag under the seat.
When V pulleys get old the V gets wider so the belt runs deeper so it is effectively turning a smaller pulley and will run slack ( V Belts don't stretch ) .
The belt also wears on the side which will make it run deeper and again go slack.
Where the spring attaches to the deck, it wears a slot or groove so again the belt runs slack.
The tensioning arm pivot never gets cleaned or lubed so it's travel get reduced and again the belt runs slack
The blade brake levers also wear allowing the brake pad to be in contact with the pulleys which slows down the blades.
A dealer is obliged to replace all of the pulleys and any item with excessive wear which gets very expensive down here ( we get ripped )
I usually fit a slightly shorter belt but in doing that you can go too far so the belt does not get enough slack to allow the blades to stop turning which is dangerous.
Being a repair shop, I have access to a lot of belts ( keep around 400 in stock ) so it is easy for me to keep swapping belts till I get the right size.
To a retail customer , belts are a non-returnable item so it is a bit harder for you to do.
Add to that non standard sized belts can be very expensive, a 1/4 " longer belt can be twice the price of the standard ( whole inch ) length belt.