Hi - I’ve been looking through the forums and been trying to find information regarding this. It’s not a big issue or anything but when doing any flight, VNAV can be enabled for descent BEFORE reaching the TOD waypoint. Should I be descending when VNAV can be activated or should I do it when I reach the TOD waypoint? Thanks 
Welcome to the Community!
You want to turn on VNAV around 1-2 minutes before TOD (often approx. 15-20nm). You can see the time remaining if you set VNAV in the Status Bar (its in the last column)
Personally I never care about the TOD waypoint itself, and often actually remove it. It’s Simbrief generated and 99% of the time completely off. Just ignore it and descend with your VNAV as early as it tells you.
Additional tip as I can tell you’re using Simbrief: Simbrief generated altitudes on your arrival procedure are always inaccurate, they are too steep and keep you too high. What I recommend is removing the Simbrief procedure and altitudes before it entirely and replacing it with the IF specific procedure.
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Adding onto what @BennyBoy_Alpha said here’s 2 cutícula things about VNAV
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Do NOT and I repeat DO NOT arm unless you know you’re going to be able to monitor the descent meaning only arm it once you get close to the TOD and NEVER leave it armed overnight, you’re better off waking up to overshooting your destination than to over speed violations.
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Before arming it, make sure to check your flight plans altitude restrictions, some STARs have really close altitude restrictions with little space in between them resulting in really step descents which makes slowing down much more difficult. Trust me, the last thing you want is needing to descend from 20,000ft to 8,000ft in just 50nm.
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12000ft in 50nm is perfectly reasonable…even 30-40nm is enough for that height change.
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Really? I think anything more than -2,5000ft/min (where the VS blinks yellow) is less than ideal but that’s just my preference
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Essentially you can turn it on as soon as you reach cruise or just before TOD. Only problem if you have it on and your not active by TOD, it’ll begin descending without you.
I just turn it off if I’m away from the device or a long haul, just in case 
I turn in on personally 1 minute TOD(Top of Descent) so the aircraft can prepare for the right -VS so descent isn’t to steep.
Started doing that now. Works out better
Glad to help😄
A lot of the time you simply need to slow down before starting the descent. When VNAV gives you a crazy descent rate, it’s often because you’re doing 320kts when the STAR is designed for an aircraft doing 250kts or 230kts.
Often the STAR will have speed restrictions, and often IF will show an altitude that’s actually a minimum or maximum, so it’s good to look up the approach plate online.
I have basically stopped using VNAV because it’s too easy to overspeed. Manual altitude and V/S control is way safer and allows you to control your speed more easily.
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I’ve never have a problem using VNAV at TOD. I activate it one minute before TOD so I don’t forget and the plane has time to adjust for descent. If my descent rate forces me to use spoilers, then I do so. If my next waypoint is below 10,000 feet, then I slow 1-3 minutes prior to 10,000 feet based on how slippery the aircraft is. IF has less track miles than what SimBrief may calculate, so you may end up with a more rapid descent rate. If that’s the case and you want to play it safe, enable it before to your liking. There’s no real wrong or right answer imo to this. It’s just a matter of how you want to proceed; start early and slow for better control, or descend later and quicker because you know how your aircraft handles descents. I say this because I use the 737 family, a320 families, etc., and I don’t have to worry about TOD for one second with VNAV because they are very tolerant of quicker descents.