Castor oil is very viscous and sticky and so is best mixed with lighter oils.
I use a teaspoon of Castor oil in a cup of Avocado oil and various herbal extracts to make a root stimulating pack (which I leave on all day).
However, you have to try different quantities and variations to find out what suits your hair.
The greater the quantity of Castor oil in the overall mix, the harder it will be to wash out.
Also, I find Castor oil quite drying, so be careful to mix it with rich emollients if either your hair or scalp is dry.
Have fun experimenting.....