Javascript division

Hello all, I use this script for calculating a division, but the answers are all over the place.  Can someone tell me what codes would work better?

var price = this.getField("design.1")

var qty = this.getField("Grand.1")

event.value = qty.value / price.value

Sometimes it calculates correctly, other times it doesn't.

I'm using Adobe Acrobat X Professional.

Thanks for the assistance.