I didn't make any cuts or "darts" in the mat anywhere to get it to lay flat, but I did cut off a sizeable chunk at the top front of the mat (like maybe 6 or more inches). I believe the cut line shows up best in the 2nd & 3rd photos below. There was much more material above this point before it was sliced off. Even where I cut was several inches above the factory mat, so there's a lot of flexibility depending on if you're using a factory firewall pad or not.
Here's what the mat looked like when first spread out in the cab:
Here's the cut when first installed. I liked the way it almost looked like a factory curl or bead:
And here's what it looks like today:
Previously I had a diamond tufted pad covering this section, but I came to prefer it the way it looks today.
For comparison, here's how far up the firewall a factory mat extends:
