SPM describes the needle movement, not the whole job
A piece includes acceleration, trims, color changes, thread breaks, hooping, loading and unloading.
Measure minutes per 1,000 stitches
Run representative approved designs and divide elapsed sewing time by stitch count. Keep cap and flat measurements separate if they differ materially.
Then add operator handling
A machine can finish a design before the operator prepares the next piece. The slower of machine production and operator supply becomes the effective rate.
For multi-heads, apply head utilization
If only three of four heads are loaded on a partial cycle, the fourth head contributes no output.
Use good pieces, not attempts
Rework and rejects reduce deliverable capacity. Include an allowance based on your own records.
Use your own measured data. These guides explain the decision structure. Run the calculators with your actual stitch times, handling times and batch sizes before treating a machine class as a fit.