Looks like the OP's hand wasn't steady. Use 2 hands. Preferably use a rest of some kind under your arm. Think about target shooting. You are far more stable with 2 hands and a rest than with the gun (MIG gun or firearm) held in one hand. If you shake sideways, you're welding on cold metal. Up and down; screwing up the wire feed rate. Maintain the correct stick-out.
Most smaller machines have "relative numbers" for voltage and wire speed. Mostly because it's expensive to control those variables to specific values. The settings from the chart on the welder are "starting points" Best settings for your welder may be a good bit different. Try having someone gradually change the wire speed as you weld a bead. Then compare the best speed to the chart. Pull the trigger and watch the wire feed. It should be smooth and consistent. If the wire speed varies the weld varies.
As for duty cycle, my machines weld fine 'till the overtemp sensor shuts 'em down. The smallest machine we have is 180 amps so it may not be a good comparison but Miller makes top of the line stuff so I wouldn't expect the weld quality to deteriorate as you approach max duty cycle.