Speech to Text Advanced: recognizing the letter “X”

I quite frequently use a title name called Jax-620, and most of the time speech to text recognizes it as Jack’s 620 versus the alternative. Every once in a while, I am able to get it to capture the “X”, but I have not found out what I am doing that is effective and ineffective.

For example, while I was dictating this now, I said X just now but above it typed out the lowercase word “ex.” Wondering if there needs to be a hard long sound or what I need to do for this to get through.