If you won't mind exact number of kilos (e.g., 8000) displaying with a trailing decimal point (i.e., 8. k), then you can use this custom format...
[>=1000]0.####, k
where you would put as many # signs as maximum decimal places you are willing to display.
If you would not mind trailing zeros (e.g., 8.000 k), you could use this instead...
[>=1000]0.000, k
where you put put as may zeros after the decimal point as the maximum number of decimal places you want to always display.