Maybe do in vba to set that counter in your Events table based on PhlebotomistID and BloodDrawDateTime(make sure this is a datetime data type). Add PatientCount to Events table. Maybe something like below could run either once a day or in AfterUpdate of the form to do this every time a record is created?
Dim db as Database, rs1 as Recordset, vsql as string, vPhlebID, vCounter as Integer
Set db = Current Database
vsql = "Select PhlebotomistID, BloodDrawDateTime, PatientCount From tblEvents Where (BloodDrawDateTime = Date()) Order By PhlebotomistID, BloodDrawDateTime"
Set rs1 = db.OpenRecordset (vsql)
rs1.MoveFirst
vPhlebID =0
vCounter = 0
DoUntil rs1.EOFrs1!Edit
If vPhlebID = rs1!PhlebotomistID then
rs1!PatientCount - vCounter + 1
Else
rs1!PatientCount = 1
vCounter = 1
End If
rs1!Update
rs1!MoveNext
Loop
Or better yet maybe do this before saving new record:
vPatientCount = DMax("[PatientCount]", "tblEvents", "[BloodDrawDateTime] = #" & Date() & "# And [PhlebotomistID] = " & Me.PhlebotomistID)
If IsNull(vPatientCount) thenrs1!PatientCount = 1
Elsers1!PatientCount = vPatientCount + 1
End If
Syntax needs adjusting but that's general idea.