Is there a secret to fitting the abdomen?

You may not even think to ask this question, but I find that cutting the fabric a bit generously through the center-front seams beginning about three inches below the end of the bust helps give the comfortable belly-forward look of some period art sources. The Parement Master, who painted "a wedding" c. 1380 as seen in Margaret Scott's A Visual History of Costume: The Fourteenth and Fifteenth Centuries, provides an ideal example of the belly-forward silhouette. The Limbourg Brothers, who painted most of the books of hours so famously commissioned by the Duc de Berry in the early 15th century, also painted women with ample bellies.

By "cutting the fabric a bit generously" I mean either flaring your panel pieces on the bias or adding triangular gores at the point where you want the flare to begin. There is no hard and fast rule about which method is more authentic for this period's clothing, but one advantage to using gores set in to straight-grain seams is that you will not deal with a bias-to-bias stretch/sag. I've done this both ways and find that as long as you let your gown hang for a day or two on a dress form before finishing all the seams, you can comfortably use the bias-to-bias construction without much problem with sagging seams later. This is also dependent on the type of fabric you use — thickness, weave, and fiber content.

I like to fit the center-back seam closely until the point at which the backside begins its flare. The side seams can be fit just as closely to the hips, or you may want to flare the fabric from about three inches below the bottom of the bust. One advantage to flaring the side seams at this higher point is to gently disguise any back rolls you may have.

The center-front seam can be closely fitted through the torso until the hips, or like the side seams, can be flared from a point at least a few inches below the bottom of the bust. In effect, you would be creating a falsely high waist, but there is nothing unattractive about that. On the other hand, if you feel comfortable doing it and want to achieve a more fitted look throughout the length of your torso, do not be afraid to fit closely all the way to the hips. This look is certainly found in period art too. An example can be found in Joan Evan's Dress in Medieval France, plate 57, which shows sculptures of female donors to the Abbaye du Trésor, c. 1400, all wearing gowns fitted through the torso. The choice really is yours, depending on how you want your body to appear and what style you are reproducing.

The following diagrams show two generalized profiles dependant on the shaping of the side and front seams: