The reheat pressure is generally kept within 20% of the initial pressure of the steam. Why? What factors must be taken into account in the design consideration of a boiler? What is basically needed...

The reheat pressure is generally kept within 20% of the initial pressure of the steam. Why?

What factors must be taken into account in the design consideration of a boiler?

What is basically needed for maximum overall economy, i.e. the boiler running at maximum efficiency but at lowest cost?