Congrats, NS.
My two euros worth for OT and GB.
OT: The Boeing 737 FMS warns of the computed top of descent point and prompts for 'reset altitude' at a variable point BEFORE computed TOD which depends on speed and forecast descent winds plus actual cruise wind. It is around 8-12 miles, I think, from memory, so around 1-2 minutes, for the 737. NORMAL operating practice (for economy) is to use the FMC for descent profile. The crew will probably ask for descent at the 'prompt', and reset the altitude window on the control panel (which 'allows' the autopilot to start the descent at the computed point). This is where the problem can arise, as they MAY then wait for the system to start descent, which is probably not what ATC want. Hence our suggestion to use the words 'NOW' in the clearance if that is what you want. I know it is not correct, but it WILL prompt the 'computer operators' to start descent either by reverting to 'old-fashioned' autopilot techniques, or by asking the FMS to begin descent early, which in the 737 is at 1000fpm until it reaches the pre-planned descent profile.
The 'level by' restriction (if known to the crew - see previous comments about the 'secret' ones!) will be pre-programmed. If another is given by ATC, it will be entered at that point, and the Boeing FMS takes about 15-20 seconds to re-compute its descent profile. Again, 'Descend NOW to be level at (new restriction)' alleviates the problem of the delay.
GB: This problem normally arises when the crew have a higher than normal (hence steeper profile) programmed descent speed OR strong head-winds which will both delay the ECONOMICAL descent point (and in the case of higher speeds, make it NOT where you have seen the others start). The FMS WILL achieve the target, but if it gives you a missed heart-beat or two, back to the use of the word 'NOW' might help to alleviate the worry? At least you will see some descent begin. Alternatively give a desired rate of descent which is a common feature in the Frankfurt approach sectors and elsewhere.