A90 controller described a minimal spacing event on final; claiming that the Final Monitor positions lack definitive responsibilities and authority in facility documents; a factor that should be corrected.

Narrative

[I was] working Final 1 at BOS; Runway 4R in use; IFR weather with a tailwind on final and 1.5 miles of compression. A C550; was on final ahead of a CRJ1. The Citation slowed more than expected inside the final approach fix and the compression was greater than the expected 1.5 miles. I alerted the Final Monitor before the aircraft got closer than the 2.5 miles required at BOS and in time for him or the Local Controller to take action (the Local Controller had both aircraft on his frequency). The Final Monitor overrode Local and heard him telling the Citation to expect minimum time on the runway due to an aircraft minimally spaced behind him on final. The Final Monitor chose not to coordinate with the Local Controller about the collapsing separation and the Local Controller chose to allow the CRJ1 to continue the approach. Both aircraft landed without incident. [I recommend] we utilize a Final Monitor position at A90 on occasion. However; this position is not mentioned in any directives of the facility; has no official duties and has no authority or responsibility. As a result; controllers are not quite sure who should be taking what actions when separation is starting to decay. Officially; in this situation; I should have overridden Local Control and told him to send the CRJ1 around. But because I had instructed the Final Monitor position to coordinate with Local concerning the situation; I assumed that he would take care of this should the situation continue to deteriorate. My recommendation would be that this position be detailed in our SOP; be mandated for use in certain weather conditions; be properly staffed when needed and the position's authority and responsibility clearly spelled out for all to know and understand.