You may be making several mistakes, but it is difficult to tell. I strongly suggest breaking your calculations in to steps. In the first step convert the stock concentration from mg/mL into a molarity (M). To do this you do need the molecular weight. Next convert the stock concentration into millimolar (mM). At this point you are ready to use C1V1 = C2V2, because the units of concentration of the stock solution are the same as the units of concentration in the assay, namely millimoles per liter.
Finally, common mistake in applying this formula is to use the wrong volumes. Your use of 0.464 mL makes no physical sense. I sometimes ask students to draw a picture of the dilution, so that they can more easily see what is happening.