Well, if they say descend NOW, then you would have to figure out ROD based on speed etc. But if possible wouldn't you want to delay the descent until you are at a distance roughly 3x the altitude loss?
So in you example you need to be level in 40 milen and have 10000 feet to loose:
You need 30 miles to loose 10k, (3miles per 1000') so you can wait until you are within 30 miles of assaigned waypoint, or just start to ease it down and follow up with the math until you are once again in the "slot"