To guarantee that you never miss anything, this is what I've done on a similar sort of issue:

# I'll use psuedo-code since you did :^)
while (my $line=<>) {
    print LOG $line;
    if ( ++$linenumber == 90% of max ) {
        LOG == LOG1 ? open LOG2 : open LOG1
    }
    if ( $linenumber == max ) {
        LOG == LOG1 ? LOG = LOG2 : LOG = LOG1
        LOG == LOG1 ? close LOG2 : close LOG1
        LOG == LOG1 ? rename(LOG2) : rename(LOG1)
        $linenumber = 0
    }
}
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Depending on how fast your logs fill up, you could push it up to as much as 98% of max before you bother to open the new log.