any reduction in temperature is good for the injury even if it is only on degree but the most effective temp has been found to be between 2-4 degrees centigrade. I have worked with hydrotherapy and game ready equine therapy for a while now so if you have any further questions feel free to PM me. Definately gotta apply cold for a while to be effective thoug 20 mins really.
I can't give actual temperatures however if you use an ice pack, frozen peas, or a frozen gel pack you should really have a thin towel or cloth between the ice pack & the skin otherwise you can cause damage to the skin. So wrap an ice pack in a tea towel before applying it & it will be ok.
Furthermore don't apply an ice pack for any longer than about 20-25 minutes as it ceases to have any further effect after that period of time. You should then remove it & if needed re-apply later.