Thermal co-evaporation is a promising method for large-scale uniform perovskite deposition. However, the optimisation of deposition parameters is an intricate process and often not reproducible between different laboratories. In this work, we look at the decomposition of formamidinium iodide (FAI) upon evaporation in high vacuum by tracking the composition of the residual gas with a mass spectrometer. We find that the precursor material degrades during the evaporation process into hydrogen cyanide (HCN) and sym-triazine (C₃H₃N₃), leading to an increase in pressure, which is commonly observed during the deposition of FAI. Using optical characterisation as well as X-ray photoelectron spectroscopy on co-deposited perovskite films, we demonstrate that this background pressure strongly affects the resulting film's stoichiometry. Using two different vacuum setups, we are able to show that significant changes are imposed, e.g. on the optimized co-evaporation rates by the specific vacuum chamber setup. Our results have important implications for the optimized evaporation of FA-based perovskites as they identify key issues related to the deposition of the FAI precursor.