I think you will figure that out as you quilt. You do not want the stitches so tight they are gathering fabric, as that means the thread (which will be a single strand!) will be subjected to excessive stress. On the other hand, the stitches should create dimpling in the fabric. If in doubt, you can pull a little, then smooth out (or stretch out) the sandwich along the stitching line you have just created. That will give you about the right amount of tightness.