Finding 1/V from absorbance in Lineweaver-burk plot

I’ve got six test-tubes with $0.5\ \mathrm{ml}+0.5\ \mathrm{ml}$ of 0.005 %, 0.0025 %, 0.00125 %, 0.000625 %, 0.0003125 % and 0.0003125 % starch and 4.2 pH phosphate buffer solution (2, 4, 8, 16, and 32-fold dilution of 1 % starch solution).0.5 ml of beta-amylase was added to each test-tube, and heated (without sixth test-tube). Then the absorbance of each specimen was measured (sixth test-tube used as reference) in 530 nm wavelength.

I have to find the $K_\text{M}$ of β-amylase from Lineweaver–Burk graph.There is no problem with plotting $x$ points on graph (it’s just a reciprocal of substrate concentration).But how can I use absorbance data to find $V$ that I need to plot on $y$-axis?