The calculation looks at the distance needed for the aircraft to descend in the air mass at the weight and ambients existing.
Operational data (FCOM and similar - which is built into the black box) will present suitable information for the pilot to find this.
Looking at still wind conditions
(a) a typical jet profile is flown generally at reduced thrust initially and then flight idle. This typically approximates something in the vicinity of 3nm/1000ft with a few miles to slow up and reconfigure for the approach. To this must be added/subtracted a few miles to allow for wind during the descent (which one may or may not have with any accuracy)
(b) turboprops may be a bit more flexible and will vary somewhat more according to Type
(c) unpressurised aircraft will be limited by cabin ROD so, typically have a much flatter descent path depending on speed.
The black box, depending on the accuracy of the data fed to it may or may not do a good job of figuring the descent.
The pilot, with knowledge of his/her Type and an imperfect knowledge of ambients, likewise, can make a pretty good/not so good estimate on the day.
Knowing the approximate profile required, the attentive pilot can modulate the ROD/IAS slightly on the way down to effect small changes to accommodate variations on the day. With a bit of skill and attention, the typical descent can be initiated and thrust not need to be played with until spinning up for approach.
So, for instance, years ago on the Electra, we had a short sector (HBA-LST) which we often flew with no other aircraft about. Generally was a dollar on the sector to see how close the pilot could go to making TOC=TOD with an aim then not to touch the throttles until rolling out on final. Great fun and made for a sharp mind .. all hand flown, raw data, of course.
ATC, on the other hand, can impose short term tracking requirements which totally throw the descent planning to the winds and, in such cases, the pilot needs to be able to replan on the fly. Often this will require out of sequence and normal flow reconfiguration to change the basic descent path for the aircraft.
For a large aircraft, one can play with a descent from
(a) however shallow is required by keeping thrust on, through to
(b) the typical 3nm/1000ft plus/minus a bit, through to
(c) 1-1.5nm/1000ft if the need arises.
Most jet Types have a problem with pressurisation with steep descents so that often will constrain flexibility for passenger operations.
If circumstances conspire on the day to put you ridiculously high, an orbit or two, or track stretching is used to lose height.
In summary, the black box is only an aid to doing it .. the pilot still is (should be) the entity driving the show.
The previous post is a simple way of using the basic calculation for nil wind
FL300 = 30,000ft x 3 /1000 plus a bit = 90 + 10 = 100 nm which is a pretty typical jet starting point.