As noted, a very basic set up.
Where your tensioner CURRENTLY is sitting and the slop STILL in the belt, that currently installed belt is simply way too long
I would think, on the steel engagement shaft, there should be some visible "wear" on it where it goes thru the frame. If so, that will give you an idea where, approx, that shaft (and pulley) should be..
If you take a similar sized (diameter) rubber hose, wrap it around the pulleys (as it is now), you can measure the hose and get a real close idea on the length belt you'll need... Or cut the one down for measuring purposes...