If a liquid level is the only requirement then a wellbore schematic with tubing data (for collar counts), and gas properties (for velocity calculations) are sufficient. An appropriate natural gas relative density (i.e. a mixed gas) is critical for accurate velocity calculations. If a wellbore has been purged with air (78% N2), but you are assuming a default natural gas relative density of 0.6, your liquid level will appear to be below PBTD!
If bottom hole pressures are required, appendix 2 is a convenient form outlining the data necessary for calculations as per #AER G-5 and AER.pas file submissions. Note oil, water, and gas (#LNG) rates are important for fill-up ratios.
Echometer™ holds short #welltest courses around the world on acoustic well sounder technology. Their course includes far more detailed information than is possible in a blog, and is highly recommended for anybody using an #AWS. www.echometer.com.