Lettuce is a hardy crop that can be started throughout the growing season. It prefers cool temperatures, growing best at 16°C to 19°C. Approximately 900 seeds / gram.
Hints:

Plant 7 mm [1/4"] deep in rows 30 to 60 cm [12-24"] apart.

Thin to 20 cm [8"] rows.

Plant 2 to 3 times deeper in the fall for the earliest spring lettuce.

Great Lakes - 85 days. Heads are large, round, dark green, solid. Core is small. Heavy blistered, deeply cut leaves. A good market head.