Speaking of drawing lines, I personally never got into using marking knives. I have a couple somewhere in the drawers, but I never liked scratching lines in the wood. They are hard to see, and you can't erase them. I use fine 0.5mm and 0.7mm mechanical pencils ...
Hmm. I am not questioning your advice @Bruce Johnson, but in order to get this idea out of my head this might just have to be a live and learn experience for me. That's even more reason to spend little money on a marking knife.
My idea is mainly using it for marking fret slots. I use a plastic LMI fret template to mark a point on the board for each fret. Then I use a right angle metal square to mark the full line of the fret slot. In the past I've used an awl to mark that line and I thought it would be more exact to use a marking knife. I then slowly widen that line until I can get the fretsaw in it. I don't trust myself to just start sawing on a pencil drawn line (I don't have a fret sawing miter box so I like to establish a groove to set the saw in.) I know that is not ideal, but by taking such "baby steps" I can eventually get good results. Yes, it does take forever