To the best of my knowledge, there is is no law that determines the proportion between Basic and DA. Different states have minimum wages which ascertain the Basic Wage and changes therein linked to inflation. This can form the basis of determining Basic and DA relationship.
Some States also have Basic and DA, wherein Basic remains constant over a period of time whereas DA keeps changing libked to inflation.
As regards HRA %, Income Tax act lays provisions for HRA taxation which in turn form HRA equation as % of Basic + DA . %'s defer from city category.
It is better to consult a professional CA while determining salary structures.
Trust this helps.
26th October 2014 From India, Mumbai