Someone recently asked about making a pattern for a tank top with a racer back. Here is how I would do this using Garment Designer.
Start with a Contoured top, Sleeveless, with a Scoop neck and a Wide neck style & choose the level of fit you want.
To narrow the shoulder strap, select the segment at the top of the armhole and move it inwards, taking care to keep the garment shoulder parallel to the sloper shoulder (the green line). You may also want to move the point where the neck meets the shoulder over if the Wide neckline hasn’t made it as wide as you want – you want the shoulder strap to be at about the middle of your shoulder.
Looking at this garment, I decided that the back neckline needs to be moved down. I turned off Front/Back symmetry so I would not affect the front neckline.
Now for the racer back. I already have Front/Back symmetry off, so selecting the point between the two segments of the back armhole will only affect the back of the garment. Move this point towards the centre back as much as you want the cutaway portion to be. I also moved the point down to get the angle I wanted at the top of the strap.
To get a better shape armhole, I adjusted the curve control points as well.
The final pattern with seam allowances: (I would probably adjust the top of the armhole once printed out to get a smooth curve.)