0 - 2 of 2 tags for Average

HI,
I have a table that stores a timestamp field, START_TM.
What I want to do is find average number of records on this table for the past one month.
I wrote the following query and it works.
SELECT
count(*) / (select (count (distinct start_tm)) from DB_NAME.TBL_NM

I have a small database to keep track of weekly scores.  Each participant can have 2 or more scores each week.  Some players don't play every week and have no scores for those weeks.  I have the following tables:
Person:  pid, pname
Week: wn, CalDate
Scores:  wn, pid, score