I used to shoot a fair amount of night photography stuff back in the mid-1980's,and my best tip for you would be to pick an ISO setting and an f/stop, and use that as the basis for estimating exposures.
For example, pick your camera's lowest or "base" ISO setting, let's say it is 200 ISO. Pick a smaller f/stop, like f/8. And from that consistent basis, learn how long you need to expose various night scenes.
One of the advantages of a low-ish ISO like 200 and a small-ish aperture like f/8 is that it allows you time to do some off-camera flash pops, and it gives you a pretty decent amount of depth of field. One of the mistakes a lot of people make on nocturnes is to expose to quickly--the resulting photos look very,very dark,and I think it's actually better to make the exposures look a bit brighter, and then to "pull back" the highlights later, in post. You cannot totally,totally blow out the highlights, but when all a picture shows are the brightest light sources, I consider that an underexposed night shot.
After practicing at 200 ISO and f/8, pretty soon you'll be able to estimate scenes where you need 15 seconds or 30 or 60 or 120 seconds, and so on. Keeping the ISO and aperture consistent is the way I was taught to shoot nocturnes, and it really helped simplify things.