CRA is prepared to consider that “substantially all” can be less than 90%

Some cases such as Watts and Keefe have found that in particular contexts something in the neighbourhood of 80% could satisfy a "substantially all" test.  When asked about this, CRA stated that it will "consider each case in its particular context to determine if a threshold lower than 90% could satisfy the test."

Neal Armstrong.  Summaries of 11 October 2013 APFF Roundtable Q. 3, 2013-0495631C6 F  under s. 248(1) – small business corporation and s. 110.6(1) – qualified small business corporation share.