tach instrument tin cutting

[Kaptain Kadian]

Hey guys, I am installing a tachometer into my c10 that does not currently have a tach. I have the tach and am in the process of painting the needles. I was wanting to use my existing tins and just cut the holes in them to allow for the tach to go through. Has anyone here done this and have an idea of what size hole goes in the inner tin? Or, does anyone know a good way of lining everything up to make the cuts? Thanks.