There are some great charts in the back of Alan Revere's Professional Goldsmithing. Check your local library to see if you can go there and copy just the charts. Our public library has multiple copies.
In the edition our library has, table A-8 is the "Length of Ring Blanks" and it is a table that has ring blank lengths for rings size 1 thru 14 (in 1/4 steps) sizes with metal thicknesses of 0.8 mm thru 2.6 mm. You find the ring size you want down the side, and the metal thickness across the top, and where those two lines intersect you'll find the length of the ring blank in MM. So if you put in the example Angela used and find the size 5 1/2 row, and the 1.3 mm column, where they intersect you find 54.4mm.