I do pantos in the middle of the quilt with custom borders all the time. Just takes a little more time. I use a piece of blue painter's tape on the plastic shield over the pantograph to mark the edges of the left and right borders (a whole strip of blue tape makes sure I know where that edge is so I won't go beyond it). When I get close to that right edge, I will stop (needle down) and peek over to the front just to make sure I'm in the right spot. I always check where my stitches are at the end of the row before I tie off just in case I need to go further or even take out a few stitches. You've got to keep checking where your border edges are for each pass unless your quilt is perfectly square or you have it perfectly straight on the frame. Shift happens. Good luck, it gets easier with practice.