Yay, a dirted tank! ^^ About 1" of dirt capped with 1" gravel is correct. If your gravel is quite large in diameter, you might have to cap with a bit over 1" and remove some gravel once the dirt becomes completely waterlogged, if you want. That's what I do, since I have larger gravel yet prefer a thin cap for easier planting.
Hairgrass should love the dirt but I'm not so sure you have enough light for it. I'd really like to see if it survives in your tank.