df -k is always in 1024 blocks: that is KB -- always. plain old df is in 512 block sizes (1/2 KB).
You could probably work out some kind of alias that would do the math for you, but for the most part a ballpark figure is sufficient. So, if you do a df -k and get a return for a filesystem of 2555904, I would count that as 2.5 GB, even though is is actually 2.4375. 2.5 is close enough. And I'd count a readout of 294912 as about 293 MB, even though it really is just 288.