This is a common misunderstanding. You don't need to 'cook' ketamine
All you do is evaporate the water that contains your ketamine so that it leaves your ketamine behind which you can scrape up for consumption. And to evaporate water you can use a variety of heat sources (double boiler, microwave, hair dryer, dessicator, etc).
So the temperature shouldnt be much of an issue because the water will always be boiling at 100c and wont get hotter even if you add more heat (it will just boil faster). Just remove from heat source once it has evaporated.
While I like to assume that everybody who posts here is aware of all harm reduction aspects of the different substances, I anyways feel compelled to comment on how I hope that you do realize that ketamine has it's peculiarities in terms of safety. For example during the anaesthesic effects of ketamine and due to the possible loss of coordination one needs to be in a physically safe context. Also its important to be aware of habit-forming quality of ketamine that some people experience where they use it regularly and as you know specially long term regular usage of ketamine can be toxic resulting in bladder issues and other potential problems. Lastly, be aware that sometimes there are shortages of ketamine and a flooding of cheaper synthetic alternatives which may have different safety profile and dosages (MXE, MXP, N-Ethyl-Ketamine, Deschloroketamine, etc), so it's important if you can to test your sample with simple reagents (mandelin and mecke for ketamine), TLC, or send to lab analysis.