Markets drift portfolios away from target. A bull run inflates equity allocation; a drawdown deflates it. Without a written rule, behavioural drift dominates: you sell after losses and buy after gains, precisely backwards.
The simplest workable rule is calendar-based annual rebalancing with a 5% drift threshold. Once a year, on a fixed date, you rebalance any sleeve that has drifted more than 5% from its target weight. That's it. No market timing, no tactical overrides.