As close as possible to the centre of mass of the frame, so that it doesn't make the bike respond in any unexpected or unusual way
It's <1% change in bicycle mass, and we can assume that the rider and/or team will train thoroughly with the ballasted bike before riding the record attempt/championship race.
In that case, below the center of mass to provide a stabilizing moment to keep the bike upright when accelerating a big gear at the start of the race (or reduce rider effort required to stabilize the bike).
It's <1% change in bicycle mass, and we can assume that the rider and/or team will train thoroughly with the ballasted bike before riding the record attempt/championship race.