Calculating a Field

When I run this in either edit or add mode can change the Miles field, I get nothing in the mile_pay field.

This is in the Client Before

var ctrlMilepay = Runner.getControl(pageid, 'mile_pay');
var ctrlMile = Runner.getControl(pageid,'Miles');
ctrlMilepay.setValue(this.getValue() + (.58 * ctrlMile.getValue()));

return false;