Leeb-12

Hi there, I have a Thundercat which is running hot when stationary (i.e. in traffic). The fan doesn't kick in until well into the red which I understand is common but it has actually boiled over before now. I am going to change the thermo switch for a lower operating spec switch so just canvassing opinion on what would be the optimum temperature for the engine to run at please? Any information greatly appreciated, Cheers, Lee