The quality of decision-making in public policy is closely linked to the quality of democracy and institutions, where accountability and transparency belong among crucial determinants. There are always at least two parties involved in the public policy process - alongside the decisive control sphere (policymakers), there is also a sphere of influence (interest groups and lobbyists). Transparency in lobbying outreaches a single law: lobbying rules - in any form - are just one piece of enhancing transparency in political decision-making. For this purpose, we have introduced a new methodological approach for evaluating transparent lobbying by creating a comprehensive and universally applicable tool for assessing lobbying transparency - the Catalogue of Transparent Lobbying Environment (CTLE). CTLE is divided into four categories: the first two relate to policy actors, i.e., lobbyists representing the influence sphere and decision-makers as targets of lobbying; the next category is the so-called sunshine principles referring to measures on the transparent decision-making process generally, and the last category relates to rule enforcement and sanction. Those proposed measures in the CTLE include 158 indicators divided into 16 groups. Most indicators are evaluated at a binominal scale of “yes” or “no”. Nowadays, we are facing the challenge of determining the significance of each measure (16 categories) for fine-tuning already existing indices of the quality of democracy/institutions. We have conducted a pilot study based on the Delphi method among experts and got preliminary results. The essential measures of transparent lobbying appear to be a register of lobbyists, disclosure of senior public employees, legislative footprint, open government data, political party funding, freedom of information, and sanctions. To increase the robustness of these preliminary results is necessary to broaden the range of experts from academia and practice involved in assessing the importance of each group of indicators.