[chaley]

I don't have a paperwhite so I cannot test it there. Testing it in calibre using a custom column built from other columns, the following works:

program:

sidx=field('series_index');
i=format_number(sidx, "{:02d}");
f=format_number(sidx, "{:04.1f}");
isInt=cmp(f, i, '', 'yes', '');
final_sidx=finish_formatting(
    test(isInt, i, f),
    '',
    ' ',    <---- There is a space between the single quotes
    ' '     <---- There is a space between the single quotes
);

# Add together the final result
strcat(
    field('series'),
    final_sidx,
    field('title')
);

It produces the following:


If the kindle sorts differently then all I can suggest is that it is doing something strange with the numbers, for example ignoring the decimal point and what follows.