When ready to finish cooking the chowder, drain the whole clams through a fine-mesh strainer set over bowl.
Reserve the clam juice (you should have about 1 cup; add water if necessary to make up any difference).
Combine the potato mixture, reserved clam juice, and the remaining 1½ teaspoons of salt in a large saucepan.
Bring to a bare simmer over medium heat (bubbles should just begin to form along the edge of the saucepan), stirring often (it will take about 10 minutes to reach a bare simmer).
Reduce the heat to medium-low and cook, stirring often and adjusting the heat to keep the chowder just below a simmer, until the potatoes are mostly tender, about 5 minutes.