PMP Practice: Modify schedule as needed based on methodology

Ash Tanaka is managing a municipal water treatment facility upgrade for Cedar Tanaka Authority. The project uses a predictive approach with a 14-month timeline. During schedule planning, the engineering team provides optimistic duration estimates of 8 weeks for the filtration system installation, while the procurement lead warns that specialized components historically take 12-16 weeks to arrive due to custom manufacturing requirements. The construction supervisor suggests 10 weeks is realistic if weather cooperates. Ash needs to finalize the schedule baseline for stakeholder approval next week. The project charter explicitly states that missing the regulatory compliance deadline will result in daily fines of $50,000. What should Ash do to establish the most credible schedule baseline?