NO! Don't back it off. My sources specify 80 foot pounds of torque on the bolt. If you don't get it tight enough, the flywheel key will shear. The key is only for alignment. The flywheel is held in place by being tightened onto the taper of the crankshaft.
If you want to torque the flywheel properly and want to do so without special tools - fill the cylinder with oil (kerosene will do), torque the bolt and when done, take the spark plug out and crank the engine over slowly to get the liquid out. NOTE: KEEP ALL SOURCES OF IGNITION AWAY FROM ANY FLAMMABLE LIQUIDS BEING USED! With most of the liquid out, crank the engine with the recoil (prefered) or electric starter to expel most of the rest of it. Depending on the liquid used, you can use compressed air to displace more of it. Finally, with the spark plug out and the engine at bottom dead center, let it set for a day or two before trying to start.