What is the temperature needed for a reflux using a mix of solvents as the reaction medium?
Trial and error, but most often the boiling point of the lower-boiling solvent.
The absolutely correct answer would be to look at a phase diagram, determine whether you have a positive or negative azeotope (or non at all), find out where on the phase diagram you are and then derive the temperature. That would tell you approximately at what temperature your mixture will boil – after you factored in boiling point increase due to dissolved compounds.
In practice, you need to set your oil bath to some 5 or 10 degrees higher than the boiling point of the lower-boiling solvent anyway. So you can just ballpark it, selecting a temperature that would keep the lower-boiling solvent refluxing. Come back after 15 minutes, if it is not refluxing, increase by 5 or 10 degrees.