Not gonna make a difference. Drill that thing at .0.080", and it'll run. Well. You're splitting hairs, and worried about minute things, but you're not considering the amount of gas that the .308 Win round can produce.
You know this already, math wise. You're 0.072~0.073" right now, on 16" barrel that you're gonna cut down by 1.5". If you're worried about it, and don't want to be over-gassed, then just split the difference. 72/73 thou on 16", 81 thou on 13.5", and you want 14.5". Run it up to 77 or 78 thou and be done with it. It's rocket science - but it's not. There's plenty of gas in this cartridge. I'd bet it'll run just fine, right now with it's 72/73 thou, cut back to 14.5"...
For the dwell time discussion - you ever seen what a rifle-gas 16" .308AR looks like? There's not alot of dwell time, and you cut that gas port at 0.100". You're overthinking dwell time on the 308. Let's say you go too big, and jack it all up - add an adjustable gas block and cut it back down.
Now, since we're just spewing out info for you, drag your ass into the Intro Section and tell us about yourself... It's right here...
https://forum.308ar.com/forum/22-introductions/
My $0.02. Take it for what it's worth.