What is the test for determining whether a co-felon was a proximate cause of the death of an accomplice?

"To be considered a proximate cause of [an accomplice's] death, the acts of the defendant[ ] must have been a 'substantial factor' contributing to the result. [Citations.]" (People v. Caldwell (1984) 36 Cal.3d 210, 220, 203 Cal.Rptr. 433, 681 P.2d 274, emphasis in original.) "Decisions in cases involving conduct of more than one co-felon acting in concert reflect the settled view that the extent of an individual's contribution to the resulting death need not be minutely determined." (Id., at p. 221, 203 Cal.Rptr. 433, 681 P.2d 274.)