The SBA recently released (5/15) the first version of the PPP forgiveness application (linked below). There are certainly going to be tweaks to it going forward, but it does provide specific guidance and calculations for a number of open issues such as: alternative payroll periods, paid vs. incurred, calculating eligible payroll and non-payroll costs, full time equivalent (FTE) reduction calculations, etc.
PPP Forgiveness Application 3245-0407 SBA Form 3508 May 2020