Average variable cost designated as the primary proxy for marginal cost, with alternatives permitted upon reasoned determination. These regulations prescribe the cost concepts and procedural approach for computing cost of production, defining multiple measures including average variable cost, total cost, total variable cost, total avoidable cost, average avoidable cost, long run average incremental cost and average total cost, and direct that average variable cost will generally be used as a proxy for marginal cost while allowing the Commission to adopt alternative cost concepts in specific cases with reasons recorded in writing.