system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] How do captains pick a warp speed?

How does Picard, or anyone for that matter, decide at what warp speed to travel? Obviously there are times when they need to get somewhere in a hurry and he says, warp 9 or maximum warp, but the rest of the time he just seems to pick a number at random. 

He rolls a warp 6.5 

Is there some guide as to recommended speeds that he's following? Is there a speed that is the most efficient?

*Edit*

I did not ask this myself, but it was taken from elsewhere.
gpt
It was always weird to me that the captain was deciding the speed.  Seems like that's the navigator's job.

"Lt. Smith, Starfleet wants us at Starbase 12 in two days. Set a course."  
"Aye aye, sir." *does a bunch of math* "Helm, come about to 185 mark 3, warp 6."