example:

PRI = 191

To get the Facility
Divide the PRI number by 8. 
191/8 = 23.875
The whole number part is the facility.

To get the Severity
Take the whole number part 23 and multiply by 8 and the product subtract from 191:
191 - (23 * 8 )= 7

PRI = Facility 23 and Priority (7)

Work backword to check our work:
23*8 = 184 + 7 = 191