You seem a reasonable individual. Please don't be the 'how dare you question me' brigade.
Great reaction. Particularly since A&C is right, and thus right to question you.
If you apply the 3X rule, what you're essentially calculating is a certain glide slope angle. A hair over three degrees, to be exact. This is an easy descent in all types of aircraft. However, the faster the aircraft flies, the higher the rate of descent will be.
Since A&C mentions a DR400 (though not the type), let's assume 95 knots. Using the 3X rule this works out to be 527 fpm. Very doable in a non-pressurized aircraft.
A larger aircraft at 300 knots can use the exact same formula but would end up with a ROD of 1660 fpm.
Now the norm for ATC for pressurized aircraft might be a 1500 fpm descent, I don't know. I don't fly pressurized aircraft. But if you apply that norm and want to calculate your TOD point based on ROD instead of a 3 degree glideslope, then dirkdj's calculation makes more sense to use than the 3X rule.