Read the ratio as a relationship
A 2:1 gather ratio means two inches of fabric are eased into one finished inch. A 1.5:1 ratio is quieter; a 3:1 ratio is fuller. Neither is automatically better.
The same ratio looks different in crisp lawn, slippery voile, medium linen, and bulky fleece. Test a short section when the fabric is expensive or the seam will carry several layers.
Calculate the gathered edge before allowances
Multiply the finished attachment length by the chosen ratio. Then add seam allowances and any joining needs. Keeping these stages separate makes the logic easy to audit.
For a 30-inch seam at 2:1, the gathered edge needs 60 inches before allowances. If the piece is split into panels, add a seam allowance to both sides of every join.
Prototype the bulk, not only the look
A beautiful sample can still be hard to sew into a waistband or bodice. Fold the test section into the full seam stack, stitch it, and check whether it presses and bends as the project needs.
Sources & further reading