I am counsellor for 1 company only. 35% of my billable time is retained by the co., as per our contract, and is allotted for overhead. Can I claim that 35% as overhead?

I assume that you are reporting income o the business form T2125 . In this case yes, if you have mandatory deductions from your earnings you would report 100% as inocme and  you can report overhead  costs  as management and administration expenses on T2125.