As far as I understand it, there is no formal calculation for "below market" rent. The term "below market" implies that households receive a subsidy on "market rate" rental housing (i.e. housing that is rented in the private market with an assumed profit motive) if they qualify based on their income and family arrangement/personal circumstances.
The way this is generally calculated is that "affordable shelter costs" are assumed to be 30% of your after-tax income. This is also referred to by the CMHC as 'core need'.
Another way to determine housing affordability, if you don't have a person's income handy, is to use a standard measure of poverty, like the "
low income cut-off", and to infer what rental prices would have to be to still qualify as "affordable" using something like the core need measure.