Mark alignment points
Mark the same spacing along both cuts before sewing. Use the production needle and thread, then inspect whether marks drift relative to each other.
Fit to the real seat
Place the seam where it will sit on the furniture and look for waviness after releasing the tension. If the join shifts, repeat after adjusting feed or relocating the join.
Mingpin buying note
ACT advises consulting both fabric supplier and furniture maker about stretch and set in knits.
Mixed constructions can require a different sewing setup from either material alone.
Send the seam location and ask for both representative cuttings.