Inverse Flash NanoPrecipitation (iFNP) is a scalable technique for encapsulating hydrophilic molecules such as peptides and proteins in nanoparticles at high loadings. These nanoparticles, which have a hydrophilic core and hydrophobic corona, may be incorporated directly into microparticles for sustained release applications. Use of iFNP instead of the traditional double emulsion process can enable higher loadings and encapsulation efficiencies. We have developed an extraction-based protocol to rapidly evaluate the impact of iFNP formulation parameters on process losses without the added complexity of producing microparticles. Among other parameters, the external osmotic pressure and, for larger biologics, the crosslinking density were found to impact the extraction process. Microparticles were subsequently produced with a target loading of 25% of a model biologic at greater than 90% encapsulation efficiency using these insights.