I'm under the impression that zeners are not very good as references. Please correct me if I'm wrong, but don't they normally have a 10% tolerance and have a large thermal coefficient? Why not use an Op-amp configured as a voltage subtractor instead?The point is, use a 15V zener for reference (you can connect at 47k resistor in series with it. Since you now have a Vcc - 15V reference, then you can always use a switcher to set the 65VDC supply to 50 or less.
Or perhaps there are no op-amps capable of running at up to 65 V?