Multiply A Currency Field With Code

Currently i have:

If CIS = False Then
CIS_Payment = "£0.00"
Else
CIS_Payment = "£20.00"
End If

which works ok but what i want to do but do not know the correct way/syntax is:

If CIS = False Then
CIS_Payment = "£0.00"
Else
CIS_Payment = "Labour_Total*18/100"
End If

Where CIS_Payment and Labour_Total are currency fields, and CIS is a checkbox updated using an update query with the same info as detailed in another table (Couldnt get the conditions to work referencing the checkbox in the other table, so duplicated the box in both tables and used a query to sync)

Any ideas???