The % of basic also depends upon wage structure applicable and policy of the company or the legal Long Term settlement signed for wages with the union. The Basic is linked up to other payments in wage structure so most of the employers would keep basic on lower side while finalising salary of employee. for example we have to pay 12% of (basic +DA) in PF as employer share so the employer cost of employment will be on higher side if the basics are high. so to maintain these employment related expenses on lower side the employer may choose to keep basic 40% or 30 % of gross salary depending upon benefits payable to employees such as PF, Gratuity, bonus etc
Hope this suffices your query. Other members can also guide you further. Please discuss this with any senior HR person in your vicinity so that you can understand this better.