Backward scheduling of WO

Hello

We have very simple requirement of backward scheduling of WO.

And Lead time is not fixed, it varies depending upon Routing types attached.

Means same Item can have different routing types which will lead to different operations and machine hours.

This will impact on start date of WO, which gets calculated even before attaching routing based on lead time.

How to have different start date of WO depending upon routing attached,

Thanks