The correct angle is critical
If you install a drain or waste line in your home it works on gravity. The correct angle is critical for them to work properly. A 1/4″ downward slope for every 1 foot of pipe is about right. Without enough angle the water and waste flow to slow and may back up, too much angle and the water will flow too fast and the solids may get left behind and cause a backup, so try to stay in the 1/4″ slope per foot range.