There are two places it can be used in this case. If all you want it to change how it appears on your display, you’ll use the transformation in the State Description Pattern. If you want to change the Item’s state to be 90 instead of 0.9 you’d do the transformation on the Channel config or if that isn’t possible using a Transform Profile to apply the transformation on the value between the Channel and the Item on the Link.
Since you are using the MQTT Binding, add a JS transform to the “Incoming Value Transformations” field of the Channel config.
Usually by defining the second Item, the Proxy Item. Then create a Rule that triggers when the “raw” Item changes that multiplies the new value and updates the Proxy Item.
On a Linux installation it’s /etc/openhab/transform. The file needs to end in “.js” and the contents will be
Then on the Channel in that transformation filed you would have
JS:nameoffile.js where “nameoffile” is what ever you named the file.