https://www.allaboutcircuits.com/projects/diy-synth-series-vco/ on his AAC used to calculate

I understand that the transistor Ic equation has exponential properties, but what equation did he use to calculate the required expo voltage? given the linear input voltage and frequency required. I want to know this since I am trying to include an exponential voltages in the code of a micro controller for a MIDI to CV to then power a Synth. Thanks for any help