The ground glass on the prism with the standard 1:1.375 markings

The existing lines make a 16mm x 22mm rectangle

To get a 1:1:85 ratio you need 11.89mm x 22mm

to get 1:1.78 you need 12.35mm x 22mm

with a fine pencil mark 6mm from the center on each side.

This gives you a 1.83 ratio on the inside of the pencil mark and about a 1.78 ratio on the outside of the pencil mark.

Line the square upto the new marks on the prism.

Use a fine pencil to mark the ground glass.

New markings on the ground glass, ready for assenbly.

Insert the prism back into it's mount and insert retaining spring.

Ready for the camera.