The calculation for capacity is based on two variables. Lost time and Standard Work. In my experience projects are created with fluff (Lost time) or with some type of buffer to meet the schedule. The fundamental reason why this happens is because people do not want to manage the project. Managing takes work and effort and this is why lost time is built in. This is incorrect. If you want to know capacity you have determine your standard work. No, I know you cannot possibly observe a 6 month project but you can observe a part of the project. For example, if you observe several days of standard work and you determine that you have 15% lost time and then you can apply that 15% to the entire project. You can then determine you capacity.