Ratios math

[alberto fuentes]

Im making a video at 2560x1080 at 60 fps. Ratio 21:9

Im importing a clip that is 1980x1020 at 30fps. Ratio 16:9

I had to go the clip properties and force ratio to 12:9 so its full
screen. I found this 12:9 ratio manually by trial and error. No idea
where it comes from, but it fits the screen and others ratios dont.
Where this ratio come from?

Also, I thought this was happening already, but this made me think
that I also have to force the fps to 60, so the clip isnt half as
short?

I mean, the clips is 2 seconds when i reproduce it, and 2 seconds in
the time line, so I think it didnt shortened. But it feels
inconsistent behavior. Does the fps gets transformed but not the
ratio? what else doesnt get transformed?

Thanks!

PS: btw, changing the ratio doesnt refresh on the screen unless you
click on duration for some reason. Just clicking is enough for it to
refresh the new values