I love a pretty dress, but summer can be so exasperating with the prolific amount short dresses available. It's frustrating to go to the mall and walk out after trying on a bazillion dresses that are just two or three inches too short. A permanent underlay is so much more convenient than an embellished slip because you never have to worry about whether it's hanging too far down or worse, if it's going to fall off!
Cutting your underlay in the same shape as the dress (in this case, a semi-circle) ensures that it will hang with the same cut as the dress and ultimately, look like it was originally part of the design. If your dress has more of a straight bottom edge, you'd want to imitate that as well. I generally cut the pieces two inches longer than needed for the finished length to accommodate a rolled hem on the bottom and the extra allowance needed at the top where you attach it to the dress.