Подсчитайте флажки и отмеченные, например (воздушный код):
Private Sub ButtonCheck_Click()
Dim Control As Access.Control
Dim Total As Integer
Dim Checked As Integer
For Each Control In Me.Controls
If Control.ControlType = acCheckBox Then
Total = Total + 1
Checked = Checked + Abs(Nz(Control.Value, False))
End If
Next
Me!TotalCheckedCheckBoxes.Value = Checked / Total
End Sub