Tyre warmers advice?

[Mike145]

I'm new to touring cars and need some advice. Got a second hand set of muchmore tyre warmers with no instructions. What temperature should I be heating too? Presumably it would be different depending on track temperature? How long should they be left on for? And finally should additive be put on before heating? Might seem like silly questions but I've been racing 1/12 oval on foam tyres so all this is very new. Cheers