This is such a great question! The way that MLS calculates official square footage is by adding together all of the floor areas of the levels of the home that are "above-grade", excluding the stairs.
This means that even if your basement is developed, the floor space is "below grade".
Make sure to note in your description when you have a developed basement that there is additional liveable square-footage below grade.
At Bōde, we currently cover the cost of professional measurements that are needed to list your home on realtor.ca. This report dictates the square feet that will go on your listing. Alas, there is no wiggle room here - it is required by the industry. Rest assured all home measurements are taken the same way.