The screw on the bobbin case is for the bottom tension. Normally, one would not need to adjust this.
Try this first. Sew a normal straight stitch along the edge of your quilt, which will be covered by the binding. Look at the stitches, both top and bottom. If the straight stitch tension looks fine, you may just be moving your fabric too quickly when FMQ quilting. If the straight stitches look flat on the bottom, increase your tension from 5.5 to 6.0 and test again. If the stitches look flat on the top, decrease your tension from 5.5 to 5.0 and test again. Keep adjusting the tension, until the stitches look correct on both the top and bottom.
But if you want to test the bobbin tension, the easiest way is to do the following:
1. Put a wound bobbin in the case, threaded as normal with a short piece of thread sticking out.
2. Hold just the thread between two fingers and let the bobbin fall. If the bobbin unwinds and falls to the floor, your bobbin needs more tension. You would turn the screw clock-wise 1/4 turn at a time, until the bobbin stops falling.
3. If the bobbin does not drop in step 2, raise and lower your hand like you would a toy yoyo. The bobbin should drop a few inches each time you bounce your hand, the distance dependent upon how hard your yoyo action is. If it does not drop at all, turn the screw counter-clockwise 1/4 turn and repeat the process.
It is very important to adjust the bobbin screw only 1/4 turn or less at a time.
I hope this helps.