What is the test for proving that an attorney erred?

"To prevail in a legal malpractice action, '[s]imply showing the attorney erred is not enough.' [Citation.] The plaintiff must also establish that but for the alleged malpractice trial or settlement of the underlying lawsuit would have resulted in a better outcome. (Viner v. Sweet [(2003)] 30 Cal.4th 1232, 1244; [citations].) 'Thus, a plaintiff who alleges an inadequate settlement in the underlying action must prove that, if not for the malpractice, she would certainly have received more money in settlement or at trial.' [Citation.]" [] "The requirement that a plaintiff need prove damages to 'a legal certainty' is difficult to meet in any case. It is particularly so in 'settle and sue'

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