Ok, I'm going to try it different:
If a guy asks:
"How can I best plan a visual approach. Do you have any advise for which heigh and speed I should have a different gates? When should I start to configure?"
Is it then a good and helpful answer to explain how to best plan your TOD descend?
This is exactly the same. CDA, in this (Euro) context, is an extension of the approach. So far none of you gave any useful tips for the original poster about how to go about doing his approach.
On the contrary:
For slowing down, it's trial and error. It depends alot on the wind and the grossweight with the 737NG but I assume it's the same with the bus. In the beginning, try to slow down in level flight to get to know the energy management of the aircraft, note the distances and times needed, to make some mental rules of thumb and then start combining the slowing down while descending at the same time as you get more confident.
It's not trial and error. There are techniques you can employ. I gave my technique in post 5. I am sure other have different ones.
Flying level is also bad advice, when ATC expects you to do a constant descend for noise abatement. And by the way, nobody is requiring the CD
A to be made at idle (though of course it would be optimum). It's not a fuel saving thing, it's noise abatement. Going down with 45% N1 is perfectly acceptable, as it makes less noise than flying level with 65-75%.
What the guy is probably looking for is this:
What rate (in what height, at what distance to FAP, at what speed) should I set to enable a continuous deceleration that will ensure I reach the FAP at the prescribed altitude, at the right speed with approach flaps configured.
Such a formula won't work, as you need a variable rate, to facilitate the slowing down, according to the aerodynamic at 240-250, clean and with the different flap settings, of the plane you fly. Also the ground speed is changing, requiring a different rate to keep the desired path. Idle descend rates also differs according to the speed and configuration changes.
Hence my advice in post 5. Get a good starting position, like 5000 feet AFE at 250 knots, with 20 nm track miles to the runway (it's on the low side and won't get the original poster into trouble). Obviously you get to that point through descend planning (if ATC didn't manage your descend from 200nm out). But this is not the part the poster is asking about.
From said point onwards, it's evaluation of how the slowing down is going during the progression. And reaction to that progression. As described in my post 5.
You want to turn all that into a mental calculation?? What is needed is a good sense of TLAR, that you get from repetition practice. Or you can learn in the beginning by watching and OBSERVING how VNAV is doing it (advice post 4).
You can plan where you want to start out. The rest is reacting to the progression.
You are assuming the original poster is an idiot (thus being disrespectful), and doesn't know how to plan a descend. I am not, as he is actually asking about the part (deceleration), which is actually difficult (especially for someone who might be relatively new in the business) and requires a great deal of anticipation.
----different topic----
If you need to start a decent "NOW", use the minimum directed/legal descent rate until you intercept the VNAV profile (which will likely be very close to the traditional 300 ft/NM guideline, should you lose the VNAV profile indicator).
"ABC123, keep a rate of 2000 fpm or greater until out of FL250".
P.s.
I do feel that descend planning is important too. It just have no relevance for the topic at hand.