That is not the way to do it especially if this is a batching system. the 4-20ma (here I'm assuming this is from a level transmitter i.e. 0-100% and not load cells. You can still use kg (or imperial if that's your flavour) so for example, when the vessel is empty you set the 4ma level to = 0kg or if you prefer 0.7% as 0kg. Then you fill the vessel to what you consider the 100% value by using a calibrated flow meter or by weighing in batches totalling them up we will say for this example 1000kg so the scaling in the plc is 0-1000kg.
then it directly calibrated in kg. for example 1 litre of water = 1kg conveniently, however, other liquids or powders will have a different value volumetric wise to kg. so depending on the medium will require a conversion factor i.e. 0.7 for some liquids compared to water. Using a level sensor as an amount in weight is not a good idea unless the medium is always the same factor. Another problem is that most vessels will have a conical bottom and this poses two problems, 1 the level sensor (assuming pressure type) would not be at the very bottom due to the outlet port will usually be at the lowest point, So it would probably be to one side therefore there is automatically a dead zone. 2. As the vessel will probably have a conical bottom to allow correct drain, this poses a problem that it will be non linear for example the bottom will vary in weight depending on the level in the conical area so as a pressure type level or rod would measure in lets say mm at the very bottom 10mm of liquid would say weigh 1kg but further up the vessel the area is greater so 10mm would equate to 1.3kg so there is non linearity due to the shape of the vessel. you could add correction factor in chunks of 0-100% from a table to correct for this. So to put it bluntly if you want accuracy then there is a lot of work to do to get level into the equivalent weight. The vessel shape, agitator or other equipment i.e. breaker bars, baffles and so on will depending on their location create errors in the conversion. Weight is weight. but level will not translate directly into weight unless the vessel is built with a flat bottom and perfectly spherical from top to bottom. If the accuracy is not a real issue then yes it will work but I strongly suggest you convert it directly into kg even if you have to add correction for 100 points (maybe less it will depend on the shape of the tank).
Take for example a silo, 3 metres in height the bottom will be conical for 1 metre, therefor the level every 10mm will have a different weight to volume ratio if the rest is the same diameter then you only need to correct the bottom 1m. If the batch is always the same size and the ingredients were constant then you could correct but this is going to take some working out.
You really have not given any real relevant information on any of the above so there are many assumptions that have to be made. What is the 4-20ma signal from ? what shape is the tank, different sized batches ? accuracy required ? Etc.
