The monthly allotment, as an enforceable cap
Enter the period's hours as a T&M hour-cap budget on the retainer. A soft limit warns the account team, a hard limit blocks further entries, and the threshold you pick — say 75% — fires the alert well before the allotment is gone.
The retainer's monthly hours become a T&M hour cap: soft limit warns, hard limit blocks, threshold alert fires at the percentage you set.