This paper investigates the demand and capacity management in very low level urban airspace, using the notion of Reasonable Time to Act (RTTA) from the European Concept of Operations for U-space (CORUS) and its Urban Air Mobility (UAM) extension (CORUSXUAM). We develop methodology for determining optimal RTTA values which balance early airspace reservation versus the flexibility of last-minute adjustments. Our analysis addresses the trade-offs between early deconfliction (which may lead to inefficient airspace use due to unnecessary reservations), and late deconfliction (which implies reduced predictability for uncrewed aerial systems operators). We incorporate weather uncertainties, using simulations to quantify the impact of varying RTTA lengths on airspace congestion and delay costs.