One thing I learned working on my Shift projects was that dear Mr. Worth, of The New Butterick Dressmaker, likely has a very good point when he writes “It is necessary in dressmaking at home to have a perfect duplicate of your own figure on which you can try your clothes as you make them.”
I found it very hard to get the dart size and alignment right on the first try, particularly for the back darts. The trouble is that while I can try on a dress-in-progress and check it out in the mirror for the front side, but twisting around and trying to analyze the back while peering over my shoulder and reaching around to pinch up excess fabric is not always so easy.
For my knitting projects, I made one of those do-it-yourself duct tape dummies, and I find it helpful for things like determining if I’ve knitted a long enough body before decreasing or binding off for armholes. However, the duct tape dummy is a squashy, lopsided sort of a thing, and far less than “a perfect duplicate” of my own figure. For one, duct tape stretches. As soon as I started stuffing it to make it stand upright, it began stretching out of shape. It also squashes, and a bit of pressure here or there easily makes it lose a shoulder or a breast (sad things to lose in the middle of a clothing project, indeed.)
I find The New Butterick Dressmaker‘s suggestions very interesting. In the era before dial-out injected plastic forms covered in flocking, the proposed formula was to buy a dress form one size smaller than your own body, and then sew a muslin form like a tight-fitting dress that fits you closely. Then pull the muslin over the dress form, and pad it out with rags or “wadding”. That way you can customize not just the overall dimensions but also the particular shapes of the various parts of your body, right to asymmetries and unevenness–“prominent hips or a round abdomen.” I might like to try this.