Hey Chris, below is a different approach which might be a bit easier to work with.
1. Add '*' to the end of your texts layers source, by hand, or by expression to text source text.sourceText + "*"
2. Apply Text Animation to the text layer.
3. Set 'Units' to be 'Index', based on 'Characters'.
4. In 'Range Selector', to start add text.sourceText.length-1. To end text.sourceText.length
5. Add 'Scale' and 'Position' properties to animator.
6. Adjust their values to properly fit position.
7. Alternatively, you could adjust anchor point position of the group, but above option will be easier and more convenient.
Now if you want to add '*' only at certain time, you could affect this by adding animation.