We recommend saving promo packs for your programmed events in this way:
Friday Night Magic—30% ~3% per week
New Player Events and Prizes—10% (Bring-a-Friend Reward)
If you run Magic events above and beyond those listed here, feel free to support those events with promo packs and adjust these percentages accordingly.
Premium promo packs are suggested for 1st place prizes in tournaments as well as for events that cater to players that want to show off their decks, like Commander.