Why large circles drift
At large diameters, one missed block changes every later checkpoint. The reliable method is to establish axes, divide the plan into quadrants, and treat every change in run length as a checkpoint.
Use numbered rows
Print or copy the row plan and number physical markers from the center line. Build the first quadrant only. At every fifth row, measure back to both axes before continuing.
Mirror, then compare
Mirror completed runs rather than independently recounting four sides. A diagonal aerial view exposes unequal flats quickly. For walls, verify the entire first layer before stacking it vertically.
Plan material buffers
The generator count covers the selected cells, not scaffolding, replacement blocks, or decoration. Carry a modest buffer and keep temporary guide blocks visually distinct from the final material.