Backtrace is harder as you don't know the current state of registers. Start from First Play and follow through. Use Open Destination to follow through (this is manual tracing). Take note of all the register value changes as you step through. Use of Notepad would help to temporarily store the register values. The debug option in step-by-step mode will help as well. Once you ynow where you want to route from and the destination route, assign the register values from the destination route to the source route and relink to the destination route where possible.

PgcEdit's trace routine is good as well. Under PGC menu, the Jump to PGC upon DVD insert macro will help you set the appropriate register values and relink it for you. I believe there's a guide for it in the homepage.