I Have A Simplicity Fashion Pro Model 640 And Need To Know The Settings For A Straight Stitch?

Just set it to A, it is actually printed on the machine in grey for the basic stitches and red for the double stitches. Or maybe your not asking the right question? Are you trying to understand the tension settings?  Those depend on the material. The basic settings are between 5 and 6 for the upper tension (the knob on the left top of the machine) and about 3 for the thread length (the knob on the right top of the machine).  But thicker or thinner fabrics will have to adjust, but you can download the manual straight of simplicity's website, it shows more basics. If you are not getting the results you are looking for it may be the tension screw on the bobbin holder or your machine may need a tune up.  And check to make sure you are using the right feeder foot, that is also in the manual you can download.