I agree with Blue , use the rule of 12 , However i always go above and beyond , the other day i did 20mls of 77/46 20mls 0/45 and 20mls 0/65 with 6 % came out really bright,this is just an example . The world is your oyster, as long as you are experienced and know what your doing with colour you can add more . If you are just starting out i would stick to rule of 12 until your comfortable ;-) xx