Using the belt tension tool is a must when changing out these belts (unless you do this for a living an know by feel what the Spec tension should be). The tool is extremely reasonably priced for the piece of mind it offer(15$ on ebay), as well as the added ability to check how worn the belts are.
This tool is the Belt Tension Gauge #91107 made by Gates
Also here is the Service manual to proper belt tension and procedure it is #13
Mercedes-Benz Model 116
A belt will only come off if improperly installed, usually insufficient tension. Or the belt snaps.