That is an interesting argument, when you consider that the U.S. was spending 4% of its GNP on national defence but was still unable to protect itself from the attack.
So you can't draw a direct link between military spending and a country's ability to deal with all possible threats. Our enemies will adapt their tactics to our military capability. If we invest in a given capability, the enemy will adjust accordingly. We have to be careful when relying on numbers to find a solution.
You are no doubt aware that the reason behind NATO's 2% average is the massive spending of both Greece and Turkey. If you look at those allies that are more comparable to Canada, you see that Canada falls within the average range. It has more to do with figuring out if we have the military capabilities necessary to fulfill our needs and ambitions.
The question I would ask is, “Do we have the equipment we need for the missions we want to undertake?” more so than, “Are we spending enough?” There isn't really a set amount.