Trend scaling can be made in many different ways, the most preferred is when the min, max, etc properties are automatically read from the PLC.
If the PLC should happen to be an AC 800M, it is common practise to use variables of RealIO type for sending data (tag info) to the 800xA HMI. RealIO variables have subproperties called .Range.Min, .Range.Max, .Fraction and .Unit which you can connect logic to, then the trend should update its scaling fully automatically. These values and connections should be presented if you go online with the application from Control Builder M. Often, the types in use are derived from some common library, if that is the case, please read the documentation that should accompany the library.
But perhaps your problem is something else or you are using another type of PLC? Please add some more data and we'll try to pick it up from there.
Fairly obviously, the configured range in the IO channel must be the same as the measurement range on the instrument transmitter.