If your PLC has trig functions, you could use the following link to calculate the volume based on the height of the liquid and the dimensions of the tank: http://mathforum.org/library/drmath/view/54105.html
If not, then a look-up table should be easy to implement. Calculate the volume of the tank for a number of heights (based on the same formula as above) and build a one on one relation between the height and the calculated volume (in retentive memory) in the PLC.