TEC temperature control is a vexed issue.
Really the best method is
proportional voltage control ...that is apply a linear voltage proportional to the temperature (cooling) your require and vary this to keep at this temperature. A PID system.
However this is not easy to do and it's much easier to use PWM.
TETech has some primers and info on TEC cooling and calculators.
Another simple method is to use a simple
fleabay thermostat controller for ~$20. Yes...crude but it managed to keep my DSLR cooler box and DSLR sensor temp to within +/- 1 degree C. Even I was surprised at this.