The distance (`r`) from the charge at `z` to the point of interest  is calculated using the following formula:  `r= d-z` where:

• `r` = distance from charge at `z` to point of interest
• `d` = distance
• `z` = point of interest

References

22.8 Electric field of a continuous charge distribution by Benjamin Crowell, Light and Matter  licensed under the Creative Commons Attribution-ShareAlike license.