There is no law to fix basic salary.
Basically, Employer see their benefit while fixing Basic Salary.
Since all the Perks as, PF,Gratuity Superannuation etc. calculate on basic only.
Secondly, Higher the salaries less the basic salary percentage works in favor of employees also while calculating their income tax.
Therefore, whenever fix basic salaries,these all factors must keep in mind, while calculating CTC basic salary play major role. You may reduce or increase CTC with the small changes in Basic Salary.
Regards
Manish Bali