A chart is trusted because it was drawn once, from measurement, and then used — not redrawn from memory at every voyage. A character described from memory at every job is redrawn each time, and every redrawing moves her coastline a little. Written once on an atlas card and clipped into the studio’s atlas, she is charted a single time, and every tool works to the same chart.
The card holds five entries in fixed order. Age, a plain number. Hair — colour, length, one habitual style. Build in two or three words. Face — one or two enduring details. Signature — the detail that survives a tight crop. Beneath the entries, the sitting line for the day: outfit, setting, hour, light. The sitting line is expected to change; the chart is not.
Paste the card word for word at the head of every prompt. Generate at the standard cost of 6 credits, keep the best third, and judge every result against the chart — not against the previous frame, which may itself have drifted off it.
Drift is not a model problem. It is a records problem. Keep the chart, and the character holds her course.