Oh thanks for your help:) I have one last question where in the default a320 will it show how much dist left to my dest?
I’m a Boeing guy but it should be in the Legs page or maybe the Prog page? I’m sure someone knows. I’m surprised the A320 doesn’t calculate a TOD for you, I know the PMDG planes and the CRJ do.
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The default A320 does put a TOD waypoint on the ND, it’s a little horizontal line then a down arrow. I have no idea if it’s calculated correctly.
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Same @IslandSimPilot I am a boeing person too but I was just flying the a320 for a bit in the past 2 days but its alright I’ll just listen to the ATC and desent
Good job wading through the math.
One good rule of thumb for rules of thumb is to sanity check your answer. 
It takes some experience to know what a “good” answer is compared to a poor one, but pretty quickly you’ll get a sense that 96 miles is a sensible answer for TOD, whereas 96,000 mi is not. 3000FPM is. 30,000 (or 300, except when you’re super far or super slow) FPM is probably not.
In a passenger jet you’ll rarely be above FL450 or below FL300 in cruise.
45x3= 135. And 30x3= 90.
So, as you approach somewhere in that 135-90NM range you should be asking yourself (and ATC if they haven’t yet obliged) when you’re going to start down.
ATC in real life is generally good (not perfect) ATC in MSFS is more in the “meh” range. In either case, you’re the Pilot in Command, so ask for what you need if it isn’t given to you.
In the real 320, on the PROG page, there is an entry down around the right side that will let you enter a field identifier and tell you the absolute distance.
Personally, you can do just as well eying it on the HSD.
Remember, this is a rule of thumb. So, ideally, you should be “sniff testing” this as you go along.
“I’m at 90NM, FL300. Time to start down.”
Then,
“I’m descending through FL250, I’m at 60NM. 25x3 is 75NM. I’m probably a little high.”
And so on.
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I’ll try all of these methods and which ever works best for me, I’ll use it always!
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Hey, I can’t remember if anyone mentioned that there’s also an “add 1NM for every 10kts” you will need to lose” rule of thumb on top of your TOD calculation.
Ex. If you need to be 250kts by 10,000ft and you are at FL400 descending at 300kts:
40,000 - 10,000 = 30,000
Shortened to 30 x 3 = 90NM
Plus,
300kts - 250kts = 50
50 x 1NM/10kts = 5NM
So, 90NM + 5NM = 95NM
It could be argued that, unless you want to use gear to slow down (assuming a pretty standard jet gear limit of 250kts and a common min “clean” speed closer to 200kts, you could also add that second 5NM to take you from 250kts to 200kts when you are ready to configure.
Same comment as before about rules of thumb. It’s just a starting point. 
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