When a defendant, charged with murder, claims diminished capacity due to voluntary ingestion of alcohol, what range of evidence permits findings from first degree murder down to involuntary manslaughter?

When a defendant, charged with murder, claims diminished capacity due to voluntary ingestion of alcohol, factually the range of the evidence on the amount and effect of his drinking often permits findings all the way from first degree murder down to involuntary manslaughter. (People v. Conley, Supra, 64 Cal.2d at pp. 324--326, fn. 4, 49 Cal.Rptr. 815, 411 P.2d 911.) Even legally the court is [27 Cal.App.3d 647] faced with a continuum, rather than a series of clearly defined alternatives: except in extreme situations no law tells us just how much effect on premeditation negates first degree murder, or to what extent the capacity to harbor malice must be lessened before the crime is not even murder of the second degree. In this case, how can we have any real confidence that the trial court correctly assessed the effect of defendant's consumption of alcohol when we know that it first required him to overcome an erroneously imposed presumption of premeditation and deliberation? ⁷