I would suggest creating a formula first and then putting it on the form in a normal font and seeing if the correct value is returned. When you get it correct then add the BarcodeUSS128 and change the font.

I've done that, however I need a FNC1 character within my formula.

To encode a control character, enter it in the format: ^000, where 000 is its digital ASCII value. For example, ABC^009 encodes ABC followed by TAB. To encode FNC1, enter: ^F1

I tried both ^000 and ^F1, however this didn't work.

I have found the easiest way to make this work is to buy the Azalea Crystal Report function:  http://www.azaleabarcodes.com/code-128/crystal-reports/

The function will do all the calculations for the checksum and has the font that will work.  I hate buying fonts, but this made it much easier.