Generally, data found is sea level static take-off power unless stated otherwise.
Mass flow cannot be measured easily, so indeed, mass flow is calculated, but not how you expressed.
Dealing with gas turbine performance data, the best tool to calculate the mass flow is using a gas turbine simulation tool (the mass flow is usually not given). If there is more data available (e.g. thrust, specific fuel consumption, turbine entry temperature), these can be used to calculate the mass flow. Gas turbine performance engineers furthermore take into account the technology level, the overall compression ratio to forecast turbine cooling flows and apply turbo machinery (fan, compressor and turbines) off-design characteristics maps so that they can make calculations of other performance points than the take-off power.
The flow in the design/reference point (the point where you have the most data available, usually take-off power) is found iteratively when tuning the model. The mass flow for any other operating point is calculated by the model based on the input parameters (ambient/flight and conditions power setting).