It sounds silly or pedantic sometimes but if you are specifying how a storage subsystem must behave, it's best to use unambiguous units for things like capacity and throughput. That way suppliers aren't confused and can't game the units.
It makes no sense to count it in 1000-based SI units. You'll perpetually mis-align the data in whichever underlying storage technology you're using, and it's bad for performance and resource consumption.
Ask for disk sector alignment; we don’t have disk sectors any more, we use SSDs with blocks. And the sizes don’t need to have a neat number of kilobytes. Just the same as a litre of water doesn’t have a neat number of H2O molecules in it.