Use A4 landscape at actual size. The dotted circle marks the cylinder, and the short line should measure 20 mm. The front edge of the sheet faces the viewer.
The simulated setup uses a 50 mm diameter cylinder, 68 mm high. The marked eye is 237.5 mm in front of its centre and 175 mm above the sheet. This template is a digital study and has yet to be tested with a physical mirror.