Backfilling nodes that have ledger gaps

One more for you: What are the steps to make horizon db backfill to a specific elder ledger number? A core node may not be fully synced and may have ledger gaps.

Ideally, sync a stellar-core from the desired eldest ledger (using an appropriate CATCHUP_RECENT value) and start a horizon instance with a clean db. Horizon will then ingest the longest segment of contiguous ledgers up to present, which should be stellar core's entire history.
• How do you determine an appropriate CATCHUP_RECENT value? Isn't CATCHUP_RECENT the number of ledger entries to include? How do you convert ledger entries to ledgers? – John Murphy Feb 7 '19 at 11:18