I belive its set by height not volumn. In order to bleed the forks you'll need to fill them up with the legs all the way compressed and the springs out, then suck the oil out until proper level is achieved. If you buy 3 liters you'll have more than you need, 2 would probably due, but might be calling it a tad close.
Yeah, I'm only guessing based on other forks as well. I've not done a ZX of that year. What you have to watch is that you will need quite a lot more oil than the forks actually take, because you have to fill them all the way up to blead the air, then suck out the extra oil...You very possibly could do it with two liters but I always buy three so I'm sure to have plenty. I do forks often enough its useful for me to have extra fork oil laying around anyway
I use a turkey baster with a small aluminum pipe and thumblock on the end. I set the height of the lock with a ruler, then insert the pipe into the fork untill the lock rests on the fork leg and draw out the fork oil untill I can't get any more. Works for me Steve's setup looks a bit nicer, but either will do the job just fine. All that matters is that you get the oil down to the proper level without contaminating it.