Consider the following example:

> 
> Now, to finish off, I would like to add one last stroke.
> 
> 
> 

If my understanding is correct, "now to finish off" is an introductory part. So there definitely should be a comma before I.

However, I am not sure if there are two separate introductory words/phrases("now" and "to finish off") or one.

And if there are two, should they be separated with a comma in formal writing or not? What is the general rule for this?