public and private replys appriciated

Hi everyone, im new to programing and im working through some things and ive come upon an isse that seems to be a common issue in these threadbut with none to clear an answer, so i thought id pose my own question.

Working with a S7 315 cpu
AI4/AO2x8/8 bit combo card and also a testing carddi8/do8xdc24v traning card with the dip switches on the front for testing.

Trying to set up an automatic control circuit in lader lang. to fill tank when its empty and to cut off at 100%. useing the AI and a 0-20ma single from a generator to give me my "levels" from the tank. so far im useing a di to act as my switch selecting auto vs manual then fc105 "read analog value" with the AI piw258 as the input 100 as hi lim and 0 as low lim 0 for bipolar so it only stays in positive polarity. (this all works so far) then the RETval on the right side of fc105 is mw102 which gives me 16#0000 when monitoring with the glasses on. and md104 on the out which is a real number scaled at 100, which is what i want. but i need to use that for a comparitor to give me the cut off on my coil at the end of the circuit showing the pump filling this tank is full. so the tank fills to 100 at anything less then 20ma and then stops filling ( denergizes the coil) when its at 20ma and its reads 100 and tanks full.

would love a reply with steps on how to fix, this, again really new to this so please no short hand, trying to learn it right and get this project done.

Thankyou,
Bart