Some simple guides to baffles:

1) Any baffle is better than no baffle at reducing swirl and breaking a vortex.
2) One baffle is about 30% to 40% as effective as four baffles.
3) A round pipe is probably only about half as effective as the equivalent width flat plate.
4) Opposed area approximates the equivalent baffle effect, i.e. three slightly wider baffles are approximately equivalent to four narrower baffles.
 
In your case, you described a 1600 mm diameter tank, which for "standard" baffles would have 4 baffles about 135 mm wide.  One baffle, 300 mm in diameter is about double the width of standard flat plates, so the width is a good starting point, bit the round pipe is less effective than a flat plate.  Regardless of the trade-offs, the baffle should be able to limit surface swirling.  The bigger issue with respect to mixing, is that the baffle may not extend to the impeller location or below the impeller.  If the baffle does not extend past the impeller location, swirling will occur in the lower portion of the tank, which will reduce the vertical mixing and diminish the effectiveness of the impeller.
 

As in most cases, where practical design allows for only one baffle, the results are a compromise in performance.

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