How many characters can a 1.44 mb diskette hold - can i assume 1440000?

If you want an exact number of characters, this one is really peculiar. A 1.44MB diskette can hold 1.44 x 1000 x 1024 characters, which is 1,474,560 characters. It doesn't make much sense, but it is correct! Trust me.

one character = one byte

While you worked out what a megabyte should be, the 1.44 MB diskette is not 1.44 megabytes. It is 1.44 thousand kilobytes, where a kilobyte is 1024 bytes as you have said. This was always a misnomer from the day that 1.44 MB drives started to be produced.