What is the test for unreasonable belief in a murder case?

In People v. Flannel (1979) 25 Cal.3d 668, 674-680, 160 Cal.Rptr. 84, 603 P.2d 1, the court recognized that the defendant's honest but unreasonable belief in the need to defend himself can negate malice aforethought and reduce the offense from murder to manslaughter. The court said the trial court had the sua sponte duty to give an instruction in future cases. But "[g]iven the undeveloped state of the unreasonable belief rule, we cannot impose upon the instant trial court so formidable a duty as to conceive and concoct an instruction embodying that rule." (Id., at p. 683, 160 Cal.Rptr. 84, 603 P.2d 1.)