ASTM D2700 method for motor octane number of spark-ignition engine fuel
16. Calculation of O.N. - Bracketing Procedures
16.1 Calculate the average knockmeter readings for the sample fuel and each of the PRF blends.

16.2 Calculate the O.N. by interpolation of these average knockmeter readings proportioned to the O.N. values of the bracketing PRF blends in accordance with the example shown in Fig. 7 and Eq 4: