You can mark the focus distances on other area of the lens, not covered by the lens .
Fanotec lens ring clamps are built with special attention to the safety of lens.
Features and Highlights
Plastic insert is placed between the outer metal ring and the lens. It is contoured to fit the lens and hence increase area of contact.
Large clamping area to reduce stress on the lens barrel, reducing the risk of damaging the lens.
Mounted close to the camera to reduce stress on the lens due to gravity of camera body
Rolling the camera/lens in any position including the popular 0, 60 and 90 degree positions
Index marks for every 90 degrees in the front and every 30 degrees at the back for easy alignment of roll of lens.
Permanently mounted to lens, making reproducible mounting possible, and much quicker to setup.
Compact and light weight
note: improper use of lens ring clamp may cause stress and damage to the lens. Manufacturer is not responsible for damage to the lens due to use of lens ring mount.
Thanks for replying nick. Transparent material on the part where the indicators are maybe? marking the lens is as good as damanging the lens.