Firstly, how is your classification of the Islamic assets in your study, presumably by contract types - which are based on Islamic principles such as Bai' Bithaman Ajil, Murabahah. (others are rate sensitivity, economic sectors, purpose, and types of customers). Then, perhaps that would help to determine the required tangibility levels. Please have a look at the account of two Islamic banks here (attached).
thanks Dr Mohd Yaziz Isa for your response! But from a balance sheet of an Islamic bank (without regarding the other details in the financial statement), Which financial ratio could we calculate to measure tangibility (approximatively)