So to find the more accurate number of moles, I would let
n = 0.00125 M*.2 L of iodide-iodate solution used = mol of IO3- instead of mol IO3- = 0.00125 M?
Then, mol of IO3-(3 mol I2/1 mol IO3-)*(2 mol S2O3 (2-)/1mol I2) = mol of S2O3 (2-)
M of sodium thiosulfate = mol of S2O3 (2-)/ L of titrant used
Thanks again.