Info: if you link that Item to a modbus data Thing with read capability … the next read poll will overwrite your “initialized” Item state anyway.
A first poll will most likely have already run before your rule, and your rule will overwrite the polled data.
At the next poll after your rule has run, the polled data will overwrite your rule data again.
It is possible to make write-only modbus data Things, if that is what you have in mind.
A tweak to your rule that only “initializes” if no polled data is available yet
rule "Startup"
when
System started
then
if ( Temp_correct.state == NULL) {
Temp_correct.postUpdate(0)
}
end