Cell formating; One decimal for values smaller than 2000

Not completely what you want but maybe it is enough
Try this format:
0,0."k"

Originally Posted by nils1401

I use #. "k" as formating for my cells. This means 921->1 k, 421->0 k.

That is not the result I get with that format.

I suspect that your best bet will be to use formulas to achieve the formatting you want, rather than using a Custom Format.

custom formatting k.jpg