4k60 footage in a 24 fps timeline will be choppy. If your intent is to use slow motion only, you can set it to 24 fps in the timeline and slow down your 60 fps video to 40% and it should be smooth (40% of 60fps is 24). If you're mixing slow mo and regular speed you will need to film in both 24fps (for regular speed) and 60 fps for slo mo only, slowed down to 40%.
Another option is instead of using a 24fps timeline, use a 30 fps timeline. You can then shoot everything in 60fps, the slow motion shots can be set to 50% speed, effectively reducing them to 30fps. You can leave 60fps on a 30 fps timeline with no choppiness. At least in my testing.
OK makes sense. I just have to do the math (which you have already done). Thanks for the advice