This is something of a Frame Challenge
And something not of a frame challenge. I think the reason you're stuck is because you're married to your map.
You see, sometimes we get way too caught up in the desire to be "realistic." I know you didn't specifically say that, but you're hanging on to a realistic map of the Galaxy. Good worldbuilding is knowing when to hold out for "realism" and when to let go and let imagination take over — especially when it's one of the small things, like travel time.
Since humanity knows absolutely squat about FTL travel, you're free to declare what your travel times will be. This is really, really important! Because when you start thinking about interstellar travel you need to realize...
The gravity well representing your planet is moving.
So is the gravity wells for any moons orbiting your planet.
The gravity wells of other planets are moving.
The gravity well(s) of your star(s) is/are moving (eeek!).
The gravity wells of each galactic arm are moving.
The gravity well (which isn't uniform) of the galaxy is moving.
And finally, the gravity well represented by the center of the galaxy is non-trivial.
And that list is an awful simplification that makes angels weep and demons laugh.1.
But the point I'm making is, you're flying from one moving target to another moving target through a medium that is, itself, moving. Worrying about how to calculate time of travel is, when you start staring the specifics in the face, pretty much a waste of time. (Every possible pun intended!)
What should you do?
In the basic need, the complexity of subway travel is very little different from the complexity of interstellar travel. Humanity has wavered back and forth between geographically-related maps (the subway map of lower Manhattan on the right, above) and abstract but much simpler to understand maps (the subway map of lower Manhattan on the left, above) since the beginning of Subways.
Some people care about their geographical location.
Most people don't.2
What they really care about is getting from where they are to where they're going as quickly and as easily as possible. Because the details of travel are far below their radar when it comes to the important things in life — like where to get the latte you're desperate for. Think about it. Whether you think of the days before flight, the days before rail travel, or the days before sea travel, all most people wanted to know was "how long will I be on this contraption?" Otherwise, the simpler the map, the simpler the process of levering the money for the ticket out of their wallets. Therefore...
Calculation of time traveled is a function of Narrative Necessity
In other words, how fast do you want to get from Sol to Rigel? Pick a number. It's relevance to reality is irrelevant. Its relevance to your story, is relevant. If you need people to get from Sol to Rigel in a week, there's your number. You now know the distance between two points and the time you need to get between them. Distance divided by time equals velocity. You're done. Your shipboard computers will be doing all the heavy lifting anyway, right? Your readers don't really need (and many times, don't really want) to know how you calculated the transit times. They don't even particularly care if they're consistent. This is because travel times (unless a critical part of the plot of the story) are really just window dressing.
Now build an abstract map of the galaxy that shows major transit/trade paths very similar to what you see with subways, mark the time between the various "stops," then pop a cream soda and enjoy looking at a map that every reader of your story will easily understand and appreciate.
And remember that, from the perspective of characters in your story, most people will step up to a computer and say, "I want a one-way ticket from Wolf 357 to Coruscant." Plug in credit card, get robbed by everyone from tourist agents to insurance brokers to the Teamster's union, and out comes the proverbial piece of paper that says:
BOARDING TICKET XH9S779WKLZFL3260SJ1 departing Wolf 357 from orbital station A-227 on February 5, 2957 and Arriving Coruscant via orbital station V-002 on March 14, 2957. Experienced travel time: 13 days. Elapsed travel time: 37 days. PADDS enhanced with the CROSSTIME protocol will automatically update to local time and adjust your medical records to reflect your experienced age. All others must visit the courtesy desk for assistance. Have a nice trip and thank you for flying with Fontain Aerospace Routine Travel!
A computer labeled "Magic Happens Here" did all the work and unless you really think you readers will be impressed, no one needs to know how sausage is made.3
1 I get the idea that gravity falls off very quickly, but someone would need to prove that a small speed bump not noticed at sublight wouldn't ruin your suspension at FTL speeds.
2 That's somewhat of an unfair simplification. People care about their geographic location. Or, more precisely, their cartographic location. What they don't care about is the time required to figure out what the geography is between where they are and where they're going. The reason abstract maps are popular (whether or not they're more popular depends on a lot of variables) is because they simplify the process of choosing a destination. They're easy to read and don't look confusing. However, the purpose of this post is not to enter the "which is better?" debate. I'm just suggesting that if you step away from a "geologically accurate" map and start looking at this from the perspective of "how do people want to figure out where they're going?" most of your problem is, IMO, solved.
3 And a good thing, too, because figuring out the distance between any two arbitrary stars is no small thing. If you really want to get lost down that rabbit hole, head over to The Astronomy Nexus, download their database, and start working through how to convert Earth-to-Everything measurements to SomethingElse-to-SomethingElse. I suspect you'll quickly discover it's more work than it's worth. Although if someone wanted to earn the gratitude of every worldbuilder on Earth, building a free online interface that returned the distance between any two arbitrary stars would probably do it. If that already exists, for the love of Glarnak, update the worldbuilding resources page with it.