After a bit of experimentation I came up with this:
If you use any Node-RED home assistant node you can use JSONata to do the calculation, and the extra function $entities() in these nodes allows you to easily obtain the sensor values you want.
Set up a home assistant sensor node (add the configuration node as required).
In the ‘state’ field, select the JSONata option (J:)
Add the necessary JSONata expression.
This is my example. The $entities(‘sensor.cb_stack_voltage’).state gets the named entity, and then the state (which is always stored as a string). $number() turns this string into a number.
For you, you will only need to change the names, and also get FP, then the final line needs to be something like
You may need to multiply/divide by 100 as fp is a percent, and you may want to perform rounding.
Doing it this way sends the value directly to HA and no other nodes are required. You will only need a trigger to call the HA entity node as required.