Ultrasonic spray coating is a promising pathway to scaling-up of perovskite solar cell production that can be implemented on any scale-from table-top to mass production. However, unlike spin-coating, spray coating processes are not easily described by a set of machine-independent parameters. In this work, in situ measurement and modeling of wet film thickness and evaporation rate are presented as a machine-independent description of the ultrasonic spray coating process, and applied to fabrication process optimization for high-performing perovskite solar cells. Optimization based on physical wet film parameters instead of machine settings leads to better understanding of the key factors affecting film quality and enables process transfer to another fabrication environment. Spray coated PbI2 film morphology is analyzed under a range of coating conditions and strong correlation is observed between spray coating parameters and PbI2 film uniformity. Premature precipitation and sparse nucleation are suggested as causes of film non-uniformity, and optimal process parameters are identified. Device fabrication based on the optimized process is demonstrated under ambient conditions with a relative humidity of 50%, achieving a power conversion efficiency of 13% in 1 cm2 area devices, with negligible hysteresis.