What is the form and description of the black circle located on a mule's foreleg, and what is the permissible amount of blood for the validity of ritual purity (taharah)?

Some scholars estimated the small amount of impurity that is pardoned as being equivalent to the Baghdadi dirham, which was a large dirham coin that existed during the era of the Messenger of Allah, peace and blessings be upon him. It is also called the "Kisrawi." It was named baghli either because it was the size of the black circle on the foreleg of a donkey or a mule, or in relation to the Persian king "Ra's al-Bagh" who minted it. Its approximate size is that of the old Saudi Riyal (37 mm in diameter), or the amount that fills the hollow of the palm. This estimation is the view of the Malikis regarding blood, and the Hanafis regarding all other impurities. However, most scholars believe that the estimation refers back to common practice (urf) and what is not considered excessive by the self.