I use pearl cotton or embroidery floss. After I tie the tie I trim the tails to the depth of the first knuckle on my index finger. That length doesn't change through the whole quilt. It is about an inch.
When I hem children's clothes, I use the width of my pinkie to space the stitches and for older children I use my thumb as a spacer.
I figure that before there were rulers and such; people had to use some sort of measure. For me it is what is handy therefore my fingers.