How do i make it so entering data in a top form places a calculated value in the sub form?

I have two table, Main, and Calc. they are both keyed to field “Reference”. Main has a field called “Data”, and Calc has a field called “Data_calc”. I have made a form off of Main, with a nested sub form out of Calc. I want to make it so that entering a number into Data results in a calculated value being placed into data_calc. I have made the following epression in expression builder in the After Update property field of the text box controlled by Data:
=Child4.Form!data_calc=(-0.64)([data]-125)([data]-150)
Child$ is the name of the sub form in the main form.
needless to say, it doesn’t work. Does anyone know how i can make it work?

thanks

BTW there are asterisks between the three parenthesied expressions, but fluther turns them into bold!