To assess the quality of a mobile app requirements document, you need to conduct an inspection sampling. First, determine the requirements defects and classify them into missing, incorrect, unnecessary, incomplete, ambiguous requirements, and so forth. Take a couple of PRD's pages and calculate the average defect density (the number of the requirements defects you'll find per one specification page). The higher the average defect density, the lower the quality of requirements for developing an app is. Use data received from your requirements defect density analysis to polish your requirements specification. By doing this, you'll be able to reduce the requirements errors and, as a result, the entire time your team spent on rework.