Change the resoultion of the number sent to MQTT

I want to send a number to my MQTT broker. The Item below send the number, but with to many zeros, currently it sends the number “1.00000000”, but I want to round the number to “1.0” to before sending it to MQTT.
I guess that this can be controlled by the :default] below. I have tried a lot of different thing (like mqtt=">[ broker:water/TempWaterUsage:state:*:% 1F]" }) but nothing works.
How can I send a rounded number to MQTT?

Number Temp_Water_Consuption “Temporary water consumption[%.1f L]” { mqtt=">[ broker:water/TempWaterUsage:state:*:default]" }

MQTT Outbound transformation using JS questions

Unfortunately I do not understand, do I have to write a Java script? There is nothing pre-configured?

That is correct.