You'd be surprised to learn then that most PE deals run on the back of literal pen and napkin models. In our process at my former firm, one of the largest megafunds out there, we would scout and model new opportunities using elaborate models, report them to the MDs who would then literally whip out a notepad or a napkin mid-presentation to decide whether the investment had potential. If it was in the grey zone, it was on us to demonstrate that it had merit, using more complicated modelling and analysis.