Hot water is preferred over room temperature water to help the borax dissolve fully and quickly.
The back of my borax box suggests mixing ½ cup of borax with 12oz of water. I have found that ½ cup of borax is just too much for a 16oz glass spray bottle. Most of it will remain clumpy and make spraying difficult. The 20 Mule Team website suggests using “a little less than ¼ cup” of borax with 16oz of warm water. I have found 2-4 tbsp to be a good ratio of borax to water to make an effective bathroom cleaner with no clumps floating around.
While I usually list essential oils as optional in my homemade cleaning product recipes, I never leave them out in this bathroom cleaner. Tea tree and eucalyptus have too many good cleaning abilities to leave them out of this one!