MTD Series 400 Operator's Manual page 7

Sight and hold this level with a vertical tree...
or a corner of a building...
or a fence post
15°
Use this page as a guide to determine slopes where you may not operate safely.
WARNING: Do not operate your lawn mower on such slopes. Do not mow on inclines with a slope in excess of 15 degrees
(a rise of approximately 2-1/2 feet every 10 feet). A riding mower could overturn and cause serious injury. Operate riding
mowers up and down slopes, never across the face of slopes. Operate walk-behind mowers across the face of slopes, never
up and down slopes.