I don't think any of the calculators are exact, I used that calculator to decide what belt to buy. The 40mm Gates belts have quite big jumps in size around the length we use ie: 912mm, 920mm, 960mm,968mm, 976mm, 1000mm.
I think my belt drive came with a 960mm belt which was too long, and as the next size down is 920mm, which seemed a big jump, I used the calculator to make sure that was the belt to buy.
As I had to drill the engine plates for the gearbox I first used the calculator to find the centers for the 920mm belt, I then found I could position the gearbox with a 244mm centre distance and the front to rear clearance between the engine and swing arm was roughly the same, that is how i came to decide on the belt length.
Once I had the belt, i positioned the gearbox at the correct hight and marked the bottom gearbox mount hole. I then removed the right hand plate and bolted the gearbox in position with the bottom bolt, I then spragged the pulleys apart to get the belt tensioned right and then I ground a point on an old gearbox top bolt that I had, and used that to centre punch the engine plate through the top gearbox mount hole. The plates were then bolted together with the engine mounting holes and the gearbox mount holes were drilled through.
When it was all bolted up, the centres were 249mm and that gives about 1 inch of play at the belt centre. Obviously doing it this way means I don't have adjusters for the belt but it seem ok without, I will probably find that tolerances in belts means the next belt i buy doesn't fit though.
So I would agree the calculator isn't much cop for a definitive answer but it was good enough for finding out which belt to order.
I think you are right about timing belts as well, square toothed belts run tighter than round toothed belts.
