بسم الله الرحمن الرحيم
Masah is not done over forehead or turban rather it is done over head. You may perform wudhu while wearing turban but while doing masah over head you should take off the turban. It is fardh to do masah over one fourth part of the head and it is sunnah to do masah over entire head.
Note: Masah shall not be valid if done over the turban. (L)
Allah knows Best!
Darul Uloom Deoband