I’ve been busy writing out the workshop manual for using a staff (Volume III) and one very important thing that comes up for me is that marking individual tesserae on there should only be something you refer to at the beginning.
You need the centre tesserae marked (and you will use this in geometric patterns all the time) but the whole point of working in the way that I suggest is so you develop your eye. You want to be able to look at a space and know how many tesserae will fit in it without having to use a visual aid. This is something that you need for both figural and geometric work.
Use a guide such as a spacer block or markings on a staff to help you but try to move away from using them when you can. In time you will be able to look at a space and know exactly how many lines will fit.