Sprintsrule,
I think then you have a sensor that detects the part, and when a part is detected, you read the length with a measurement device. If the length is within certain limits, you produce a go-no go signal. Is that correct?
It seems that you are working close to the limits of your PLC. Using the SCP instruciton can produce rounding errors because SCP does an internal division calculation. You could try using Floating Point numbers in the SCP to maybe increase the accuracy, or you could eliminate the SCP and use the raw analog data directly, and set your LIM limits in raw analog counts instead of in engineering units.
If using raw counts doesn't work, then probably your analog input does not have enough "resolution" (number of bits used to represent the signal) to accurately measure your part. The fewer bits in the analog signal count, the larger distance each bit has to represent, meaning that small distance differentials can get rounded up or down.