I want to be able to simply control the cooling tempreture of a small 40 to 60 millimeter thermoelecrtic cooler
You have to monitor the temperature. There is no "clean" relationship between input voltages and the resulting temperatures -- that'll depend on a number of factors, including the ambient temperature, the thermal mass of the object you're trying to cool, how well it is insulated from the environment, and whether it is generating heat internally.