It’s rarely the mistake (the departure) upon which plaintiff and defense battle in legal malpractice.  It is almost always the "but for" part of the equation, in which plaintiff must show a departure, but for which there would have been a better and different outcome, in which the mistake and the outcome are proximately related, with ascertainable damages.  As an example, in this case could plaintiff have won his case, but for the mistake of his attorneys?

Catuzza v Rodriguez   2012 NY Slip Op 01949   Decided on March 16, 2012   Appellate Division, Fourth Department  questions whether the underlying case could ever have been won. 
"Memorandum: Plaintiff commenced this legal malpractice action seeking damages allegedly resulting from defendants’ negligence in their representation of him in an action against, inter alia, his former employer, the Erie County Water Authority (hereafter, ECWA action). The ECWA action was dismissed based upon plaintiff’s failure to comply with discovery demands. Supreme Court properly denied the motion of defendant David Rodriguez, Esq. and the motion of defendants Noemi Fernandez-Hiltz, Esq. and The Law Offices of Noemi Fernandez, PLLC seeking summary judgment dismissing the complaint. Defendants moved for such relief on the ground that plaintiff could not have prevailed in the ECWA action, inasmuch as he failed to exhaust his administrative remedies by appealing the determination of the Hearing Officer in the prior proceeding pursuant to Civil Service Law § 72. Defendants, however, failed to establish as a matter of law that the complaint in the ECWA action would have been dismissed on that ground (see generally Zuckerman v City of New York, 49 NY2d 557, 562). Failure to exhaust administrative remedies is a defense that may be waived if not timely raised (see Matter of Punis v Perales, 112 AD2d 236, 238), and the defendants in the ECWA action did not raise that defense in their answer. Further, inasmuch as " the grounds urged for relief’ and the remedies sought in [the ECWA action and the prior Civil Service Law § 72 proceeding] are separate and distinct," plaintiff did not fail to exhaust his administrative remedies with respect to [*2]the conduct of the defendants in the ECWA action (Matter of Sokol v Granville Cent. School Dist. Bd. of Educ., 260 AD2d 692, 694). "