(1) The proportional shares of tortfeasors in the entire liability shall be based upon their relative degrees of fault or responsibility. In contribution actions arising out of liability under ORS 31.600, the proportional share of a tortfeasor in the entire liability shall be based upon the tortfeasor’s percentage of the common negligence of all tortfeasors.
(2) If equity requires, the collective liability of some as a group shall constitute a single share. Principles of equity applicable to contribution generally shall apply.