Battery manufacturers test batteries at a constant current, not a constant load resistance as you have done. A constant current is easier to measure Amp-hours.
With your method, getting an accurate result manually is unlikely because the current is changing over time. An electronic meter that can take readings every second and compute Amp-hours is necessary.
Breaking in will help 5-10%.
However, actual capacity is often 15-25% less than what the brochure says.