What is the burden of proof in a litigation malpractice action?

"Except as otherwise provided by law, a party has the burden of proof as to each fact the existence or nonexistence of which is essential to the claim for relief or defense that he is asserting." (Evid. Code, 500.) "In a litigation malpractice action, the plaintiff must establish that but for the alleged negligence of the defendant attorney, the plaintiff would have obtained a more favorable judgment or settlement in the action in which the malpractice allegedly occurred. The purpose of this requirement, which has been in use for more than 120 years, is to safeguard against speculative and conjectural claims. [Citation.] It serves the essential purpose of ensuring that damages awarded for the attorney's malpractice actually have been caused by the malpractice." (Viner v. Sweet (2003) 30 Cal.4th 1232, 1241.)