The problem you are running into is more of a design problem that should be resolved or understood before you go through programming something, just as
@Aardwizz is talking about. From the looks of your drawing, you are using a Guided Wave Radar (cable or rod) installed in a tank, and I'll assume this until you say otherwise. This application is possible, but with this instrument selection, you will ALWAYS have inaccuracies below 0.2m, as the sensor is not touching the product anymore. Now, this isn't to say it will not read something, but it will not be accurate. The problem you are also running into is understanding how the sensor/transmitter is setup. With a guided wave radar, many times these are installed in a bridle on the side of a vessel/tank and they normally trap a certain amount of product (can be drained). In this design, the 0m mark can be well calibrated with product touching the sensor, but the application is such that the tank is NOT normally empty. If you can assume the process will always maintain at least 0.2m of product, then your application will work just fine. Now, if it's expected to be completely empty, you have options to attempt to calibrate the sensor to pick up product when it's not touching the sensor (the guided wave uses the cable/rod to radiate a signal and when it hits product, the reflection generates an echo that can be picked up). I've calibrated a sensor in this manner with very good success. This is not very accurate and is not recommended, but is possible. You will just need to look at the echo curve of the radar gauge to see what is possible. From there, you can then span the instruments 4-20ma signal to be whatever you want it to be, whether that is from tank bottom or sensor bottom (0.2m).
What I would do is work with the customer to understanding the actual design and process that can help you eliminate some of these uncertainties. In many cases, a tank is storing product that gets filled and consumed. When a pump is pulling product from here, many times you want to limit how low it goes to protect the pump from damage or other hazards. In this case, the suction line should be fully covered with product. Now, if suction is from the bottom of the tank, then it would indeed be 0m, but many times (depending on size of tank), this would be a side entry and is normally a little bit off the bottom. In practice, if the product goes below the 0.2m mark, then it's possible to indicate an "invalid" level and alert the operator of such a state, and if this only occurs during maintenance activities, then this is not a problem, just an understanding of how the design is limited. From a design perspective, this would have been better with a through-air radar but depends on product in the tank of course and the price a customer is willing to pay, and again, what the process design was intended to solve.