If you want to make a 3/4 inch strap, you can cut two separate strips and add your seam allowance all the way around. Then place on top of each other and stitch together. This strap has two seams.
If you are making a strap the folds and is then stitched. This would be your formula: 3/4+3/4+1/8+ 3/8+3/8 = 2″5/16. I think that’s right. The 3/4″ represents the strips, 1/8″ the gap between the strips so that when you fold it over they don’t fold on top of each other. The 3/8″ represents your seam allowance. This strap has one seam but both edges can be stitched.
Rubber cement is not permanent, it is meant to hold the material in place for a short time, until sewn. Contact cement is permanent, it is meant to hold forever ( but never does) and has a strong bond. Some fabric based on the tightness of the weave will absorb rubber cement, so you may have to use a double layer. The best rubber cement and contact is Barge, which is sold at Tandy.