The compression curve (void ratio $e$ vs. effective stress, $\sigma_v^{\text{'}}$ for a certain clayey soil is a straight line in a semi-logarithmic plot and it passes through the points ($e = 1.2; \: \sigma_v=50 \: kPa$) and ($e = 0.6; \sigma_v = 800 \: kPa$). The compression index (up to two decimal places) of the soil is ______