a) Normally sensor data would have the sensor identification in msg.topic and the sensor reading in msg.payload.
b) The dashboard number input node will surely create a numeric value rather than a string.
If you save that to context flow.set("t", msg.payload) then it is stored as a number and you don't need to convert it in your calculation.
Similarly the sensor reading may well be a number not a string.
c) When you declare a variable you don't need to wrap it in quotes.
You don't need the quotes when you use it either : var f=42 msg.payload = f +"%"
Note that you do need quotes - single or double - round the name of a context variable to indicate that the name is a string, not the name of some other variable.
d) wrapping [f + "%"] in square brackets makes the payload an array (with just one element).
Is that what you intended?
Can you please let me know where is the correct place to store a value (min - max calibration values) so this are always available at start of the flow without active input? In a “value” File or is it possible to write form Numeric dashboard input to the default value of a variable?
There is another issue with dashboard:
no sidebar available at dashboard (sure settings to show are enabled)
dashboard node is updated (node-red-dashboard and no other one installed)
dashboard node removed / install with restart etc. no effort
Someone any hint?