Four of the more dangerous words in the Devendorf household when Meredith Devendorf was growing up were when her mother would say “I have an idea.”That’s because, Devendorf joked with an estimated 300 people at a luncheon Saturday, that her mother, Laura Devendorf, was not talking about where they should go out to eat or even buy a new appliance. She was thinking of ways to preserve the thousands of acres on her family’s centuries-old estate that centers on Springfield Plantation, even maybe expand it.The luncheon, inside a heated tent on the grounds of the estate near Sunbury, was to mark the family’s 70 years of conservation work. That work started in 1947, Laura Devendorf said, when her father, John Porter Stevens, gave nearly 12 acres on a tidal creek to a group of trustees for a park to assure blacks’ access to the “everlasting health, recreation and pleasure of those fortunate enough to live” near the coast.