I have recently purchased a second hand Tomiyama Art Panorama 617 and am confused with the focusing knob. It has markings from infinity to 90cm but there is about three revolutions in total of the knob so it is difficult to work out when it is focused at a particular distance. I am keeping it at infinity for most shots which is obvious as the bellows are fully closed but anywhere in between it is difficult to tell at what distance it is focused.

I have read on the internet that ground glass should be used with a loupe to focus before the film is loaded. Does anyone know how this actually works on this camera and also where could I find ground glass to fit this camera?

Thank you in advance.