Partial remedy: stick to easily measurable facts: number of people, budget, time to completion, number of detected bugs… Then, when your manager inevitably do

Partial remedy: stick to easily measurable facts: number of people, budget, time to completion, number of detected bugs… Then, when your manager inevitably does not believe you, ask him if he thinks you missed something (no), then ask him if he trusts you (yes), then contemplate the face of cognitive dissonance.