Is there a flow chart or a graph that DAU can provide that explains when LPTA should be used or when Best Value Trade Off should be used?
There are simple charts or graphs (one such example is at this link), but the word "simple" does a disservice to deciding whether to use LPTA or trade-off to make a source selection decision. I assume you already reviewed FAR 15.101 for proper use of the two methods. However, it's also essential to review Appendices B and C of the DoD Source Selection Procedures, which discuss the two methods in detail. It's also important that your team consult DFARS 215.101-2-70, which adds recent restrictions to the use of LPTA resulting from the NDAA for 2017.