I have no idea how CRA definitively comes up with a $$ number. My simple method is to use the equivalent miles for business trips. i.e. If I earn a total of 300,000 miles a year through personal and business revenue flying along with CC purchases, of those 300,000 say 30,000 were earned through business travel, I book business award travel to the tune of 30,000 miles.
This method ensures that miles earned using business pre-tax dollars are spent on like.