When crude oil is $100 a barrel this is equivalent to $17.2 an mmbtu, which is an important ball-point to compare the price of crude oil to that of natural gas. As an example on the 1st of September 2008 when crude oil was $116.2 a barrel and natural gas was $8.2 an mmbtu, crude oil on an energy equivalent basis was $20.05, so that natural gas was in fact being traded at only 40% of crude oil.

Crude oil parity to natural gas (100%) translated into $/mmbtu results in a slope of 0.17 (namely if one multiplies the price of oil by 0.17 one receives the price of gas if it were on a 100% parity with oil)