sub numbuckets {

  my $numfiles = shift;
  my $cdsize = 700 * 1024 * 1024;

  my $total_size = (($cdsize / 2) + 1) * $numfiles;

  my $buckets = ($total_size / $cdsize) + 1;

  return $buckets;
}

printf("Bucket estimation of %d buckets is %s for %d files.\n",
       numbuckets($_),
       ($_ <= numbuckets($_)) ? "valid" : "not valid", 
       $_)
  foreach (0 .. 5);

__END__

# My output...
Bucket estimation of 1 buckets is valid for 0 files.
Bucket estimation of 1 buckets is valid for 1 files.
Bucket estimation of 2 buckets is valid for 2 files.
Bucket estimation of 2 buckets is not valid for 3 files.
Bucket estimation of 3 buckets is not valid for 4 files.
Bucket estimation of 3 buckets is not valid for 5 files.