App Build Duration Estimator

Experienced mobile app developer (iPhone or Android) to provide regular and ongoing project BUILD DURATION "ESTIMATING" based on provided requirements documentation (usually user stories, acceptance criteria and clickable prototype/wireframes). I am NOT looking for a developer to then build the apps, more someone to give me an honest experience based opinion about build duration. Projects that are proceeded with will be put out for tender (which the developer is then welcome to bid on however but not guaranteed to get it), but this role would probably suit someone that is already employed fulltime but is looking for a risk free, additional role to supplement their income in their free time. Would be happy to also discuss/agree a bonus payment scheme whereby a bonus will be paid whenever a build comes within a certain percentage of accuracy to the estimate. The types of apps involved will vary across all App types, including games and including those requiring a webservice/api/external server so would be beneficial to have first hand experience of build projects across the different types of apps.