oh dear. I can see how this happened — in many cases they may not still have samples to derive new sums from :(
what’s the export policy for Google’s new Photos service? pretty good, it turns out
the bottom line is as follows: If the time it takes to create a dump, ? < M/2 then use ?opt = ?(2?M) – ? Otherwise (it takes longer than M/2 to create a dump), just use ?opt = M.