When I took a quilting class, we took a sample sandwich and drew lines across it at 1" intervals marking each with a number starting with 1. Then we set the tension to 1 and sewed from the first line across to the second line. There, we changed the tension to 2 and sewed from line 2 to line 3. And so on. At the end, we took the sample out and looked at the front and the back to see which tension worked best.
As for your problem: I think too that the backing is not taut enough and therefore, the sandwich is too loose. You may want to use the walking foot and make the stitch length longer. At least 3 or 3.5mm.