Interesting question my brother Salam. A good spiritual leadership must be measured on how well or accurate the teachings from the spiritual leader reflects the contents of the holy/inspired books used for teaching and worship. The principles, beliefs and practices propagated must be based solidly on the teachings in the holy books. The spiritual leader must be an exemplar of good moral behavior as reflected in the holy books. Spiritually, good spiritual leadership would be blessed with countless followers and their transformed moral life is a sure indication that the spiritual leadership is bearing fruits.
Such leadership would not be influenced by modern trends of thinking and philosophy while relegating the 'textbook' (holy books) to the background. It would now deceptively lure its followers into engaging in any vile activities that would harm humanity, or nib the holy name of the Most high in the mud, cluttered with shame.
The followers of such good and true spiritual leadership must follow the virtues of love- the cardinal quality of the Supreme Being to show that they take the precepts of their invisible father. 'It is by their fruits' or conduct that we and more importantly God would measure/judge the goodness/weakness of their spiritual leadership (members and followers).
Leadership is difiicult to measure, especially spiritual leadership. Have you thought of using qualitstive method instead as narrative theory, centent analysis or discourse analysis? Then you can analyse statements and concepts.
Mats Alvesson, Lund university has written a lot of good articles in the field of leadership theory.