FAA Advisory Circular AC 91-79 provides a Braking Action Equivalence Table (Appendix 1 - Table 1) which lists ICAO braking action codes, Mu figures and runway surface conditions and the assumed braking action for each (based on agreed upon assessment of industry representatives and the FAA). Our company uses this to calculate braking, if a braking action report from a like kind aircraft is unavailable. The FAA considers this table more accurate than obtaining a Mu report from friction measuring equipment but less accurate than a PIREP.
That should provide you with guidance on braking action based on runway contamination.