A 100% effective, although unconventional, way to ensure that you never go over your indexing limit is to limit how fast the index can run.
$SPLUNK/etc/system/local/limits.conf:
[thruput]
maxKBps =
To figure out what the # should be, divide the daily license cap (1GB: 1073741824 bytes) by 86400 (seconds in a day), to get your max Kbps rate (12427 bytes/sec, or 12KB). This doesn't sound like much, and it isn't for a single second, but if splunk runs steadily all day long, you'll get close to your limit, but not go over it.
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