3 x Thousands of feet +10 then allow for wind (on a B737). For a B747- descend when the FMS says. So lose 25,000'- allow 85 miles (with slow down) or 75-80 miles keeping speed up. You would probably find this is a little generous- you could reduce this by 10 miles and increase speed if altitude too high.
It would be useful to say type of aircraft! Jet, turboprop, widebody?