Assume A=k*c. k combines both path length and specific absorbance, as long as you are using same substance and same cuvette, that's perfectly valid approach.
Best way of dealing with the problem is to draw a plot, find the linear part, then use linear regression to find out k, or to draw a calibration curve and use graphic method to read the concentration. However, just by looking at the table it is obvious that given value of absorbance (0.518) is between 0.481 and 0.602, so the concentration must be between 400 and 500 μM.