Choose the reference
Pick one edge, face, centerline, or surface that is reliable and easy to identify throughout the job.
Mark from the same place
Measure every location from that datum instead of measuring each new mark from the last mark.
Separate size from position
The width of a part and the location of that part are different dimensions. Keep them separate on the sketch.
Check the far end
After laying out several positions, check the final mark against the overall dimension. That catches a wrong entry before it becomes several wrong holes.
The calculator is one part of the job
Use the result to support a decision, then compare it with the drawing, material, tool, product instructions, and actual conditions.