3 times track miles = FL.
Works well up to Fl 300, above that you achieve a greater ROD per NM.
Below Fl 100 with speed control then slightly flatter.
But if you steam down at MMO/VNE and have a friendly controller then leave cruise level much later.
As in the good old days and sod the passengers eardrums.
Example if you are at FL 270 start the descent at 90 travel miles. You will also need to factor in wind component.
Hugh Dibley invented a circular slide rule descent calculator in the 70s.