<p>Flex-route transit service provides a hybrid alternative between conventional fixed-route transit and demand-responsive service, particularly in low-density suburban areas where demand is spatially dispersed. A key planning challenge is determining how much slack time should be added to the schedule to accommodate deviations while limiting negative impacts on existing fixed-stop users. This paper develops an analytical framework for early-stage flex-route service planning that links slack-time allocation, operator costs, fixed-stop passenger costs, and mitigation policies. The framework derives a segment-based slack-time expression under idealized rectilinear and uniform-demand assumptions and embeds this expression in a stakeholder impact model. Two service-transition scenarios are analyzed: maintaining all existing fixed stops and reducing the number of fixed stops. The framework further identifies the fare reductions and/or frequency improvements required to offset the additional generalized cost imposed on fixed-stop users. A hypothetical numerical study and sensitivity analysis illustrate how service area width, demand intensity, slack time, and passenger value of time affect request accommodation, user burden, and mitigation requirements. Additional robustness experiments quantify the trade-off between segment-based and pooled slack, demonstrate the sensitivity of required slack to near-route and outer-edge demand clustering, and show that limited backtracking provides modest acceptance gains at higher request levels. The results show that wider service areas and higher on-demand request volumes require greater slack time, but additional slack also increases costs for operators and fixed-stop users. The proposed framework provides transit planners with a transparent tool for evaluating flex-route feasibility and balancing service flexibility with schedule reliability and passenger equity.</p>