I have that bike, and had to adjust the factory set tension as I felt it was too high. The bike came with a big Allen key, which can be used to loosen (a bit) the bolts on the eccentric bottom bracket. Another Allen key can be inserted in one of the adjustment holes, and gently tapped with a hammer to rotate the bracket in the appropriate direction. Then tighten the bolts. This was several years ago, no problems with belt tension since then.
I went for a 1cm depression of the belt at the midway point when using a finger to push the belt down. Tried the app, but gave up with it due to inconsistent results.