Normally, directions for adjusting belts and chains specify the slack or movement as the amount you can move the belt or chain in the middle of the longest span between pulleys or sprockets, using just finger pressure.
The mower will have a clutch in the drive shaft between the engine and the uppermost sprocket. tessa was suggesting you see whether that clutch is slipping. He didn't really tell you how to test for that. I would try running the mower with the clutch
engaged and the mower moving forward, then attempt to hold it back. If by holding the mower back you can make the top sprocket (the one on the engine drive shaft) slow down or stop, the clutch is slipping. If it slips it will wear out.
