Good rule on speed. I used a mental pic of 2000 per every 6 miles. You need only 2 or 3 ref points to mentally correct R/D and tendency to be high or low xx miles ahead. If flying unpressurised,the relation changes from 1/3 to 1/5 as max,not intending to pop everyone else`s eardrums. Heavier jets,(80, to me) are a nuisance because of their mass,and very slick wing that does not want to slow down. So there`s the third element to calculate in your head. I tried to keep mental track of gradient,by which this thing is slowing,and calculate as i could, how these things may meet each other at 6 miles,2000,and speed as manageable. Sure ATC gives some peas to the soup,but having something to sell,it shouldn`t be a problem.