The current organization of the internal electricity market in Europe relies on a zonal market system: the European power system is divided in bidding zones, electricity trades within a bidding zone can occur in an unconstrained manner, while electricity trades between bidding zones are limited by cross-zonal capacities. Two approaches can be used to calculate and allocate cross-zonal capacity: the coordinated Net Transmission Capacity (NTC) approach or the flow-based approach. In Central Western Europe, the flow-based approach is implemented for the day-ahead market since 2015. To avoid undue discrimination between internal and crossborder power exchanges, a minimum threshold of 70% on the cross-zonal capacity available for trade will be enforced in a near future. Such a threshold could significantly impact the socio-economic welfare and its distribution between the various actors. However, no assessment of the impacts of such a threshold has been performed so far, in particular because no convincing approach exists to perform such an assessment (e.g. security constraints and redispatch neglected). This paper develops such an approach and demonstrates its applicability on a case study, in order to assess the impact of a minimum threshold on cross-zonal capacity in a flow-based market.