If the only requirement is to send the value in an SMS, the display variables are the preferred way, since you need to set one up regardless for the SMS. If your intention is to use the linearized value anywhere else, try placing your measured low value in SI 80, your measured high value in SI 81, your adjusted low value in SI 82 and adjusted high value in SI 83. Then place your measured value in SI 84 and call SB80. The linearized value will be placed in SI 85. Attached is an example piece of code (in a little different order, but works), which i use to measure pressure from 0 to 11Bar using 0-10V analog input.
PressureLinear.U9N