I currently have my setup as line1+line 2....... = subtotal + freight = total.
individual fields (line1 + line 2...) are not the ideal structure for your database. a related lineitems table will be much easier to work with. See this simple demo file: http://fmforums.com/forum/showpost.php?post/309136/
To "show the 10%" as a separate value, use this calculation field:
If ( Not IsEmpty(checkboxField) ; UnTaxedTotal * TaxRate )
Define UnTaxedTotal as you currently define your total field.
Modify the design for total as: subtotal + freight + TaxAmount
Use a field for TaxRate. This field should look up a value from another table or auto-enter the current contents of a global field. That's because tax rates change and such a set up allows you to change the current tax rate without changing the totals calculated in previous records in your system.