What additional mark is needed when applying range markings to a glass instrument cover?

When applying range markings to a glass instrument cover, an index mark is necessary as it serves as a reference point for reading the instrument accurately. This white line is critical because it indicates the specific point on the scale that corresponds to normal operating parameters, ensuring that the instrument can be interpreted correctly during operation. Without this index mark, there would be ambiguity regarding where the range is referenced, possibly leading to misreadings and negatively affecting the safety and performance of the aircraft.

The other markings mentioned, such as red lines for maximum range or blue lines for minimum range, may be used for specific purposes or designs but do not fulfill the essential requirement of providing a clear index mark for quick and accurate readings directly related to the operation of the instrument. Calibration marks in inches are not applicable in this context, as airframe instruments typically do not use such unit measurements for visual range indicators.