Hello,

Use recall precision and F-measure metrics to evaluate your clusters according to your Threshold, each time you change the Threshold, recalculate the three metrics and when you finds the best recall, precision and F-measure, you can said that you have the optimum Threshold.

regards

You can use Rand Index,ARI,F-measures.

If the cluter labell is given to you,find the confusion matrix,and then calculate accuracy,precision and recall.