estimate a range of measures

#1
Hello everyone, I am a statistics student, I'm studying for a project in my University. I have to analyze the data, I have available the measures in millimeters corrisponding two different types of objects, of course I have already done tests to groups such as t-test, actually these organized group differ. My question is how do I estimate a threshold value in millimeters noting that the measure allows me to classify it in one group or another? I thought about using a valuation bootstrap to estimate the average with its confidence interval. I would like to know if there is a specific methodology. Thank you