Calculate the Radius of 300 mm PVC Pipe

There is no such meaningful calculation, but a "5R" bending radius (R = 5 x 300mm = 1500mm in this example) is a common design for bent pipework.

There is a table giving rules-of-thumb for friction losses of bends and other fittings in most textbooks on Chemical Engineering, among others. Try Perry, "The Chemical Engineer's Handbook", any edition, or even "Kempe's Engineer's Yearbook", any edition.