Good Morning Mudball:
Yes, the begining point being 3" below the bottom of the concrete is perfect. You can carry a 1/8" per foot fall from there to the foundation line. Then, when you get outside the foundation you need to install a test tee so you can seperate the building drain for testing. Kentucky code requires 18" cover over the sewer outside the foundation line which can be achieved with a 45 degree drop. Then the 1/4" per foot fall would be carried from there to the septic tank or other point of disposal. If your ground falls faster than that, you can make more 45 degree drops to keep the sewer 18" below the ground.