How are you setting this up?
If you enter the text 1/2 into a text field, Evaluate ( TheTextField ) should return a value of 0.5
just as though you entered 1/2 into the specify calculation dialog where FileMaker will interpret it as 1 divided by 2.
I don't know why it was reading it like that. I created an evaluate field like you said above and base the calculations of that decimal field instead and it all works now.