Emerging flexibility markets are increasingly characterised by many product characteristics, differing flexibility buyers, and a disparate set of potential Flexibility Service Providers (FSPs), including those connected to the distribution networks. The FSPs that do not have an individual schedule from previous markets (e.g., wholesale energy markets) need a counterfactual to verify service provision, often called the baseline. This paper investigates the fitness of various baseline methodologies while considering different resource, market, and product characteristics. A novel baseline decision framework is proposed. The analysis showcases that no one-size-fits-all baseline method exists while proposing alternatives for FSP types and flexibility market conditions.