The loss of runway length due to alignment is required by JAR-OPs 1.490 and an acceptable means of compliance is laid out in IEM Ops 1.490. Accountability is required for a 90 degree taxi entry or a 180 degree turnaround eg when backtracking.
The IEM has a calculation for working out the loss of runway and its dependant on the radius of turn of wheels, wheel base steering angle etc.
Performance figures usually comment on which alignement accountability has been used.
Hope this helps. Although the text of JAR Ops 1 is on the JAA website I don't think the IEMs are.