Statistically the average last frost date represents a 50% chance that no frost will occur after that date. If you'd like to decrease (or increase) the risk of your plant investments, you can use the following Freezing Date Adjustment Factor (below):
According to this factor you can expect the chance of frost in Calgary after June 4 to be reduced to 10% (15 days after May 20). Note that the graph does not provide a 0% chance of frost after any particular date! According to Alberta government records, the Calgary area has seen as low as -2 C in July at least once in the last century.
The freedom from frost in this part of the world is no guarantee at any time. For the Middle Earth Garden this is not a large concern. Large parts of the garden can be covered if a frost is predicted. But of course frost is not always predictable. Hence the gamble. My investment is small, but for larger commercial growers this risk can be serious. This is where frost insurance comes in. For this garden, frost insurance is the local grocery store!