Logo! Comfort screen problem

Hi
I'm at school trying to figure something out, and i hope someone can help, sorry if my english is bad.


The problem is I have a problem with a LOGO! simens screen, it shows only the voltage, when i want to define that as something else, like liters in a tank, and i cant really not figure out how to do it on an easy way withour haveing 2 pages of unanseesry nettworks.

Im trying to regulate a water tank with a prssure transmiter, and have to show the level on a eksternal screen, only problem is it only shows as voltage but i want it for every 0,8V to be 1 liter.

I hope you understand what im trying to imply here.
Thanks

i forgot to tell, On the screen i have the possiblity to change where i want the level to, i secsoned the tank into liters and i want to have the possiblity not only to see how many liters/ Desiliters there is, i also whant to change the water capacity with the same system.