As per Sec-59 of - Extra wages for overtime-- it is mentioned that for overtime work, any worker will be entitled to wages at the rate of twice his ordinary rate of wages.
"Ordinary rate of wages" means the basic wages plus such allowances ............... as the worker time being entitled to but does not include a bonus and wages for OT work.
Therefore, as per above it is monthly gross salary less any bonus (production incentive/bonus etc) and OT payment if any.
S K Bandyopadhyay ( WB, Howrah)