Using a chalk line reel, chalk the walls of the room hosting the concrete floor at the desired level of the forthcoming floor.
Mark the location of the drain with a stick of chalk.
Using your measuring tape, measure the distance from the foundation chalk line to the drain's location. This is your base measurement.
Calculate the maximum height of the drain by multiplying your base measurement by 1/8-inch. For example, if your base measurement from the foundation line to drain is 10 feet, with the rate of slope being 1/8-inch, the maximum height of the drain is 15 inches.
Armed with this measurement, purchase the appropriately sized stakes.
Drive four stakes around the drain, one for each major compass point (north, south, east, west). Check with your tape measure to make sure they are driven to the appropriate height.
Hammer a nail into each wall surrounding the drain at the level of the marked foundation chalk line.
Tie a nylon line from each nail to each stake. This will give you a visual guide when the concrete starts pouring.