To keep the proportions correct, Seth should use 2.5 cups of milk.
We can set up a proportion to solve for this.
1.5 cups milk / 3 cups flour = x cups milk / 5 cups flour
Cross multiplying, we get:
3 cups flour * x cups milk = 1.5 cups milk * 5 cups flour
3x = 7.5
Dividing both sides by 3, we find that x = 2.5
Therefore, Seth should use 2.5 cups of milk to keep the proportions correct.