Brake servo boost ratio and diaphagm size.

The available "vacuum" serves to limit the boost ratio, but the real boost ratio is determined by the details of the design of the boost mechanism and control valves.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.