If that is the case .... then stick with the height/level gauge ;)
I understand why you are asking this question, but considering the "mysterious" nature of suspension tuning ...why add to it?
On the other hand ...if you always set it the same, then just measure the amount that you put in/subtract the amount that you suck out with the height gauge the next time you service them. Now you will know how much to just pour in!
Hmmm .... still seems easier to just use the gauge.
I've basically got to change the springs all around, but I've also got to increase the oil height and I'm going to change from 5wt to 7wt b/c of my overly abundant mass. I was trying to find a way around buying a tool that I probably won't use again, but on further reflection, once I change the springs and oil, I'll probably do it again b/c the mystery/challenge of sus. tuning will decrease. I guess I'll see what MX Tech has the kit for.