The 2018 regulations have been simplified with a new easy PAX formula. Simply take the raw lap time and multiply it by the PAX factor for your class.

To figure out your class, simply use the online classification calculator here: http://ccdb.casc.on.ca/

As per section 3.1 NATIONAL CHAMPIONSHIP AND OVERALL SCORING of the regulations, the PAX factors are as follows:

Class PAX Factor
Open Modified 1.062
Modified 1 1.052
Modified 2 1.012
Modified 3 1.00
Super Grand Touring 1 0.988
Super Grand Touring 2 0.976
Super Grand Touring 3 0.964
Grand Touring 1 0.952
Grand Touring 2 0.940
Grand Touring 3 0.928
Grand Touring 4 0.916
Touring 1 0.904
Touring 2 0.892
Touring 3 0.880

http://www.asncanada.com/wp-content/uploads/2017/08/2017_ASN_Time_Attack_Regulations_Final.pdf, see section 3.1.3.

For example:

Raw lap time: 1:22.8, Class T3:

88.8 seconds * 0.88 = 78.144 PAX time (1:18.144)