Wha is the formula for equal tee pipe?

A common group of round pipe fittings laid out by parallel line developments are intersecting pipes, commonly called tees. These are laid out like any problem in parallel line development. However, there is one new principal to learn when laying out intersecting pipes and that is the development of the intersecting line of the two pipes. The line is called the mitre line. The pattern for the tee-piece cannot be laid out until the mitre line is developed. Therefore, developing the mitre line is the first step in laying out such a pattern. 

Main center of the tee to end tee branch will be same size