I am unfamiliar with an absorbance of 0.40%. 0.40, yes, but not the percent. If it really is 0.4%, then I would use 0.004 for A.
A = kc where A = absorbance, c is concn and k is a constant. You must calculate the k.
Use any one of the data points, for example, the first one, to calculate k.
A = kc
0.64 = k*5.5 (I wonder if that is 5.5 molar, 5.5%, 5.5 mole fraction or just what?). Calculate c.
Frankly, I would calculate c for each run you have listed, then take an average to obtain an average for c and use that.
Then A =kc.
Plug in 0.4 (if that is the absorbance), the value of k you found, and calculate c.