Check out this video to learn how to modify(offset) trimlines
Step 1- Select everything inside the socket. Use Select Region and the circumferential & Symmetrical toggles
Step 2- Invert so everything outside the socket is selected
Step 3- Modify region and build up a square edge so we know exactly where to trim (7-8mm buildup)
Step 4 (optional)- Blend, and add only to get a posterior trimline that works for your patient.