15% of salary means 15% of salary in respect of the period during the which the accomodation is occupied by the employee.It means take pro-rata salary i.e. salary only for the period of accomodation and not the whole salary.
Here salary includes:-
1.Pay (basic pay)
2.Dearness allowance considered for calculating retirement benefilts
3.Taxable allowances
4.Bonus/commission
5.Other monetary payments (not being perquisites)
But salary excludes:-
1.Arrears of salary/salary in advance
2.Dearness allowance not considered for retirement benefits
3.Allowances exempt from tax
4.Employer's contribution to P.F.
5.Non-monetary benefits or perquisites under sec 17(2)