Using the tag scaling function does not allow for the 4mA offset of my 4-20mA loop field devices. I'm guessing that this needs to be allowed for by performing a calculation. I check the calculation box in the tag properties window, this causes the IO and scaling tabs to disappear and a calculations tab is substituted. There seems to be little guidance for using this feature. Perhaps I am on the wrong track, advice please?

Ok I have this sorted thanks

I'm glad you solved this, i'm just posting for others who may find this thread.

Scaling tags is pretty simple using the scaled and unscaled values boxes. You don't need to solve for any offset in a y=mx+b style equation as that is handled for you in VTScada.

If you look at your given value for two known points and provide the scaled values for the same two points. You will find these in the tag properties for I/O tags such as the I/O and Calc tag (which is the tag type to use for most I/O and calculations).

Here is an example which I hope might simplify things:

Let's say that you have a device sending temperatures in celsius and you want to show those values in Fahrenheit.

We can use the boiling and freezing points as our known values for the scaling. In the unscaled boxes, use 0 and 100, then in the scaled boxes enter 32 and 212 and that is all you need to do.