That is not a measurement you need to know. You need to know seam allowance. On a piece of welt , that is the distance from the stitch line to the two cut edges of the fabric. If you sew two pieces of fabric together and your stitch line is 1/2" in from the ends of both pieces of fabric, you are using a 1/2" seam allowance. The original was probably 3/8". If you choose to have 1/2" seam allowances, that distance would be 1/2". That is determined by the width of the piece of fabric you make the welt from, or simply where the stitch line of the welt is in relation to the edge of what you're sewing the welt to.
In the first picture, there are two pieces of fabric to make the welt from. The one on the left is 1 3/8" wide, and the other is 1 5/8" wide. If you use 5/32" welt cord, the wider piece results in a welt with a 1/2" seam allowance. The narrower piece results in a 3/8" seam allowance. In other words, the distance from the stitch line on the welt to "A" is 3/8", and the distance from the stitch line to "B" is 1/2". You can sew narrower welts and put the welt edge in 1/8" from the edge of the fabric and still have 1/2" seam allowances.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 02-01-2011 at 11:24 AM.