Twin wall flue system design, calculating the offset height

Using different bends to make an offset (45, 30 and 15 degree) results in different flue heights - for eample using 15 degree bends means that the offset section will be much higher than if we used 30 degree bends. This is why we need to know the height at which the offset has to be complete. This is the same as the lowest height of the obstruction.