Record BOTH motion and full-time.
Often the full-time video explains otherwise unknown details about what I see on a short motion clip, particularly PRIOR to the actual trigger. HD space is relatively cheap. The difference between 2 4TB drives and 2 8TB drives is less than $300. How much video do you want/need to store? I recently helped a buddy install 12 cameras, (10) 2MB and (2) 4MB, all running 30fps, max bitrate (8192) and he's getting 10-13 days on (2) 8TB drives on a Dahua NVR.
Assuming a home system where data doesnt need to go over the Interwebs, I dont understand the need to calculate down to the last KB. As @Rakin says, you'll almost always end up wanting more HD than you initially think
The motion alerts are nice for quickly finding the action. But as @bigredfish said, having full-time recording assures that you capture everything that you need, even if it hasn't yet triggered an alert or if the movement stops for a while.
And you can have both (at least you can with Blue Iris... I can't speak for how the NVR systems work). So you have the best of both if you record full time, but also set up motion alerts.
This makes setting the motion detection a lot less critical.
HDD space gets cheaper and cheaper.
I've had events where I had a camera set to only record on motion, and ended up wishing I had full-time video for periods of time when the subject remained largely motionless or was small enough to not keep the recording triggered.
But there are some cameras in my system that I do set up to only record on motion. I need to add more HDD space, myself!
I agree with @bigredfish and @J Sigmo . I recorded motion only for awhile and thankfully moved to continuous and kept my motion detection settings just like they were. Alerts are there as they were before and I can now see what happened before or after the alert if I want to. I am using using Blue Iris. Continuous is the only way to go if you can afford the hard drives and have room for them in your pc.
As others have noted, motion detection isn’t 100% perfect. It sometimes misses stuff completely (and records nothing) or misses some stuff because it doesn’t start recording early enough or ends the recording too soon.