I don't know whether 20% off-center means 20% of the radius or 20% of the diameter, however, a 2.0 service factor is probably sufficient, provided the mixer is not operating when the impeller goes through the liquid level. If the mixer will be operating at the liquid level, a 3.5 service factor is more appropriate.

The service factor assumes that the other design factors are properly handled, including mounting support, impeller balance, etc. By the way off-center by 20% of the radius, may as well be on the center of the tank for processing purposes.