Save a large array into equal length .csv files?

Hi Guys, I am trying to save an adjusted very large data set into equal length .csv files. I am using the following script from this link with my own database:

 %%Step 1 - create a tall table
varnames = {'ArrDelay', 'DepDelay', 'Origin', 'Dest'};
ds1 = datastore('airlinesmall.csv', 'TreatAsMissing', 'NA', ...
                'SelectedVariableNames', varnames);
tt = tall(ds1);
%%Step 2 - operate on tall table
tt.TotalDelay = tt.ArrDelay + tt.DepDelay;
%%Step 3 - use tall/write to emit .mat files
writeDir = tempname
mkdir(writeDir);
write(writeDir, tt);
%%Step 4 - use parfor to parallelise the writetable loop
ds = datastore(writeDir);
N = numpartitions(ds, gcp);
csvDir2 = tempname
mkdir(csvDir2);
parfor idx1 = 1 : N
    idx2 = 0;
    subds = partition(ds, N, idx1);
    while hasdata(subds)
        idx2 = 1 + idx2;
        fname = fullfile(csvDir2, sprintf('out_%06d_%06d.csv', idx1, idx2));
        writetable(read(subds), fname);
    end
end

I am adapting the script in step 4 to the following in order to specify that each .csv file has 20000 rows:

 RequiredDataRowsPerFile = 20000;
 ds = datastore(writeDir,'ReadSize',RequiredDataRowsPerFile);

It works to some degree as there is an impact; however, the outcome does not generate an equal distribution of .csv files in terms of number of rows (of course the last file will always be different).

I would appreciate any help. Thanks

Tim