Stupid question?

I remember when all we had were red diodes with outputs of a few mW. All those early laser diodes had monitor photodiodes that intercepted the light coming from the rear facet of the actual laser diode to provide feedback for current control circuitry.

So here's my stupid question that perhaps one of you who have a decanned 445 diode can answer: What percentage of the diode's output is lost through the rear facet of the device?

Thanks.