I think I am following the thread now. Bernie gave you a sample which you are trying to use as a guide. On the first rung branch, if first does a FFU, only if R6:0/DN is ON, just to make sure that is at least one clear space in the FIFO stack.
Then on the next rung branch, you are supposed to set your Source word for the following FFL. You must do this BEFORE you do the FFL, not after as you have it now. Bernie did a dummy word by adding 1 to N7:0 and saving it back to N7:0. For your program, you need to bring in the newest Water Weight (from F34:0 ?) and save it in N7:1. (If the Water Weight has 2 decimal places, first multiply it by 100 so that when you put it in N7:1, xxx.yy will become xxxyy).