Math Is Driving Me Nuts

The business rule is, the sales manager is commissioned on the avg. number
of appointments set up per salesrep per day during the month.

I have 2 tables: The UserLog table records only 1 entry per day per user
(salesrep). This will log how many salesreps worked a particular day. The
second table logs any appointments set up.

UserLog: ID, UserName, EnteredTime
Appointment: ApptID, EnteredTime, ApptDate

I figured that, for a given date ranged, I could
1. sum the number of appointments
2. sum the number of days worked
2. sum the salesreps / number of days = avg number of salesreps per day
3. number of appointments / avg number of salesreps per day = avg number
of appointments per sales rep

But this logic is flawed. If I average out every day and then take an
average of this daily average, I get a different result. Any ideas on how
to best solve this problem?

Thanks.