This is always going to come down to the base hair colour you are working on, the best thing to do is do as many different variations of skin tests on yourself as you can so you become familiar with the colour ratios and how dark they will get on the skin and hair. Generally, when henna is warm on some clients it cools shortly after the application anyway. You can apply some brow powder to tone the warmth in the meantime if required. Some clients do not have the problem of warmth with henna.