The most important thing to note here is that your measurements at the start make sense - for example, it's safe to assume that the maximum base width (D) is similar, if not identical to the lower sofa base width (L2) measured from the back.
Otherwise, this means there's some kind of taper. However, the front can be slightly wider usually so don't be surprised if there's a bit of discrepancy. We'll let you know if we find that the measurements don't make proportional sense with the photos provided.
The last tip to note, is to stick the measuring tape to the sofa instead of having it float if you want a more upholstered look.
If you're going for a relaxed look with a skirt, then your vertical measurements can be perpendicular to the floor.