How do you calculate the recession velocity?

$$
begin{alignat}{7}
&& frac{Delta lambda}{lambda} & = frac{v}{c} \[2.5px]
&therefore & v & approx frac{Delta lambda}{lambda}c
end{alignat}
$$
If you want to find the recession velocity of a galaxy which has an absorption spectrum shown below, the $Delta lambda$ is the same for all the changes (approx from 400 to 430, 410 to 440, 590 to 620, etc.), so when using the equation how do you know which $lambda$ to use as the rest wavelength because they all give different results?

$hspace{150px}$