According to weins modification of the working length determination if bone resorption is there then:
Wein modification of working length determination by radiograph: 
1. Reduce length by 1 mm if NO root + bone resorption 
2. Reduce length by 1.5 mm if bone resorption 
3. Reduce length by 2 mm if both root and bone resorption
Reduce the length by 0.5mm
Reduce the length by 1mm
Reduce the length by 1.5mm.
Reduce the length by 2mm.