To obtain obstetric conjugate, which of the the following factor should be subtracted from the diagonal conjugate?

Obstetric conjugate cannot be measured directly with the examining fingers, it is obtained by subtracting 1.5-2 cm from the diagonal conjugate. It usually measures 10cm or more. Diagonal conjugate is determined by measuring the distance from the lower margin of the symphysis to the sacral promontory. It measures 11.5cm. Obstetric conjugate extends from the middle of the sacral promontory to the middle of the symphysis pubis on its inner surface a sho distance below its upper margin. It is the shoest anteroposterior diameter. Ref: Varney's Midwifery By Helen Varney, page 1208