3 times the altitude to lose works for me in an MD-80 In your example a .74/280 decent would be 30*3=90. 280 to 250 would add 3 miles. If you decend faster you need less. subtract 10% for a .79/310 decent. In your example (30*3=90)-10%=81miles. Add 6 to slow from 310 to 250. 87 miles. You can also adjust for tailwinds and headwinds. Add 1 miles for every 10 kts tailwind and subtract 1 for every 10 headwind.

I flew the EMB-120 for 4000 hours and can't for the life of me remember how we started down. I do remember about 30% Tq to get zero thrust but as for miles. I cant remember ever doing a 3 to 1 calculation. Huh