In principle, it is not desirable that public funds are used to pay for private failures.
Multiple recent studies (Cline, 2017) trying to derive the optimal Common Equity Tier 1 capital (CET 1) ratio for banks have arrived at figures on opposite ends of the spectrum – Dagher, Dell'Ariccia, Laeven, Ratnovski, and Tong (2016) estimate the optimal CET 1 ratio of 9-17% of the risk weighted assets, Admati and Hellwig (2013) estimates optimal CET1 ratio to be 36-53% of risk weighted assets.