For a farmer planning the harvest of crisphead Great Lakes lettuce, the most useful estimate would be the average time frame of 85 days (the midpoint between 80 and 90 days). This average can help the farmer better plan for optimal harvesting time. Additionally, it is important to consider factors such as weather conditions, soil quality, and pest management, as these can impact the actual time to maturity.
To ensure a successful harvest, the farmer should also monitor the growth of the lettuce as the estimated harvest time approaches, looking for signs of readiness to ensure the best quality and yield.