What a great way to re-purpose a useless bit @JustJim.
For those unfamiliar with a marking knife, you might like to let us know what you are using it for and a little bit about them. In the past, I've used a marking knife where I needed an extremely fine crosscut on a piece of timber. I marked deeply with the knife first and then cut along the line. Pre-cutting the timber grain with the knife helped prevent any splinters from tearing out from the circular saw.