Email Spelling Confusion: Canadian vs. American Pronunciation

Interesting - when i spell an email to Lindy's calling action on the telephone, I use the Canadian pronunciation of "Z", which is "zed" and it understands that as z-e-d but when i pronounce it the American way "zee" it understands that as the single letter z. I tried adding this to the Context in the Agent Settings with the hope it would handle this better, but no success: "Canadians pronounce the letter Z as "zed", so don't spell out "zed" if they are spelling something to you." Any suggestions on how to get the agent to understand "zed" vs "zee" when saying the letter "z"?