Diagonal drag lines from armhole to bust on every button-up I make Fit Help

Drag lines point at the problem, radiating away from the place that needs more room. Lines running from the armhole toward the apex mean the fabric is being pulled toward the bust because there isn't enough length and width over it, which is the classic sign you need a full bust adjustment even though you cut by full bust measurement.

That's the trap. Commercial patterns are drafted for a B cup difference between high bust and full bust, so if yours is bigger you get a garment too big in the shoulder and neck and still tight over the bust. Cut by high bust, then FBA the difference.

Start with a 2cm FBA on a toile in cheap cotton. And measure your high bust properly, snug, under the arms and above the full bust, because that number does all the work.

The dart the FBA adds can be rotated almost anywhere afterwards, into the side seam, waist, gathers or a princess seam. The volume is what matters, not where the dart ends up.

Worse when you raise your arms is a separate clue worth checking. A low, wide armhole restricts arm movement and drags the whole front panel with it. Compare the armscye depth to a shirt that fits you well. A higher, smaller armhole is more comfortable, counterintuitively.

Photograph yourself in the toile from front, side and back with your arms down and relaxed. Reading drag lines in a mirror while holding a pose is how people misdiagnose fit, because you unconsciously stand differently.

Also check the grain hangs straight. If the centre front isn't perpendicular to the floor you can chase bust adjustments forever when the real problem is that the front is riding up.