That sounds like an overly complicated method. I use my 3 times table. Take the distance you have to lose - 10,000ft in your example. Drop the zeros and take 10 times 3 which gives you 30 track miles to lose the height. Take your ground speed and halve it but this time add a zero. So with a ground speed of 250kts you want a rate of descent of approximately 1250ft per minute. I usually add on another 10 miles to slow down to configure and possibly another fudge depending on whether there is a large tailwind or not.
What type are you flying? If your flying something with an FMC then putting in altitude/speed constraints to give you a visual clue on your profile helps as well.