The Actual Overlap differs from 25% to evenly space the nozzles across the header, as the number of nozzles required is a whole number.
Complex spray coverage patterns such as large circular/rectangular areas, specialized nozzle patterns such as Even Spray Descaling Nozzles and prediction of coverage for spray injected into a gas stream or pressurized vessel are all examples where this calculator is not useful.
Spray Coverage is based upon Theoretical Coverage and doesn't take into account the collapsing effect that gravity exerts on a pattern for very high spray heights. Theoretical Coverage is considered to be accurate for spray heights up to ~6 feet. At these heights and higher it is suggested that you contact a spray nozzle specialist and review your requirements with them.
The Effective Coverage Overlap is a suggested minimum and based upon using Flat Fan Nozzles or Full Cone Nozzles having a parabolic (tapered) spray distribution.