Oh yes 
I’ve overlooked totally the function of the freewheel, actually the process was the opposite as above mentioned…however.
I do have a (mechanical) tension meter for belts - but it does not at all help!
I’ve tested also frequency belt apps but they did not work, I believe that is because the tension is too low to measure for them; these apps are likely made for industrial purpose.
There is no correct amount of tension known for a Strida!
(In the manual, Ming cycle is giving us just one dimension - that is not sufficent! We need two dimensions; deflected distance AND the required force to achieve this deflection. We’ve told Ming cycle several times…)
You can adjust the belt only by feeling, sorry to say that.
Mark Sanders said somewhere: “Keep belt tension as low as you can tolerate!”
I’d add this: “But keep tension as high as required.” (To avoid tooth skipping.)
For my meaning; if you can touch the frame with the belt - then, tension is too low.
To get a feeling for this procedure I’d recommend that you try too much tension intentionally.
Without riding the bike, pedaling with your hands, you can feel that the whole drive train developes drag. Now, adjust the excenter a bit below and feel the difference.
For a better orientation, to see how much rotation of the excenter has which effect on the belt, you could make a few marks (on adhesive tape) at the excenter and corresponding on the frame.
I’ll repeat here also once again that the belt tensioning method, shown at the Strida manual - without using a hook key - is wrong!!!
Only a tight groove nut AND a tight bottom bracket bolt TOGETHER are able to withstand the high forces at the bottom bracket!