mileage form - what am I doing wrong!?

I'm trying to create a mileage calculation form that mutiples "milesdriven" by .56.
I don't want a separate column with ".56". I want the math to take place behind the scenes, if you will, and automatically populate in the designated field. 

I've used the simplified field notation with what should be the correct formula... and I'm not getting any calculation in my field. Am I not putting the calculation in the correct field's properties? Is my field notation incorrect? This is maddening. It seems SO SIMPLE, and yet...

Screen shots included.

I want the calculation to populate in the field named "x 56_1" (and so on)
Here's what I'm doing:
Properties for x 56_1 field >>calculate>> simplified field notation>> milesdriven*0.56 >> close >> preview >> type number into "milesdriven" field >> no calculation in the "x 56_1" field (!!!)