// Set float value with full precision
begin
  advstringgrid1.FloatFormat := ''%g'';
  advstringgrid1.Floats[1,1] := 1.23456789;
  advstringgrid1.Floats[1,2] := 9.87654321;
end;

// ensure floating point number is only displayed with 3 digits
procedure TForm4.AdvStringGrid1GetFloatFormat(Sender: TObject; ACol,
  ARow: Integer; var IsFloat: Boolean; var FloatFormat: string);
begin
  FloatFormat := ''%.3f'';
end;

// do calculation at full precision
begin
  advstringgrid1.Floats[1,3] := advstringgrid1.Floats[1,1] + advstringgrid1.Floats[1,2];
end;