Neither NIC nor the American Corrections Association in their accreditation standards support staff/inmate ratios as a measurement of adequate staffing, post coverage, or supervision of inmates. NIC does not make any recommendations in terms of staff/inmate ratios.
The process for determining adequate staffing for a jail facility, and make it defensible, is to conduct a staffing analysis on a facility by facility basis. There are simply too many variables such as physical plant design, level of security, level of programs and activities, state and local standard and statutes, etc. to recommend a specific officer to inmate ratio.