For a 'normal' jet, i.e. one that cruises 4000 to 5000 feet above the Mach/CAS changeover altitude, 3 times the Height plus 10 miles works quite well.
For high flyers, (Learjets et al) operating well above the Mach/CAS changeover altitude, 2.5 times the Height plus 10 miles would works better. (the Mach phase of the descent is an accelerating phase, and quite a bit steeper than the CAS phase).
The +10 miles is to allow for deceleration for approach, and if your aircraft has a high flight idle thrust, you might find that +20 works better.
Regards,
Old Smokey