um...ok, first what are you making? i ask because oil tanned can be kinda soft and may curl if you braid too tight. anyway braiding over flat is the same formula as braiding over round. 1 1/2 x circumference divided by how many strings used = width. now if you are using precut lace then instead of dividing by string divide by your lace width to get how many strings to use.
example---circumference is 3/4" (.75) x 1.5 = 1.125. divide this by 1/8" (.125) = 9 so by formula 9 strings would be required to cover, you could try 8 it might cover, depends on you braiding style. formula for length is always 1 1/2 x needed plus however much you need on each end to finish your project.
the challenge in braiding over flat is keeping your lines absolutely straight, as deviation will be more obvious than braiding around a round core.
hope this helps you.