capital gain

I had purchased plot area 600square feet  for RS 80000/- in 1999-00, In the year  2005-06 i constructed ground plus 1st floor for Rs 5.4lakhs.  In the same year i have sold half of the 1st floor i.e 1 room for Rs 5.4lakhs.Total area of the 1st floor is 846square feet.I have also incurred  RS 40000/- for cost of flooring for ground & 1st floor.

i want to know what will be the cost of acquisition for room 1 which i have sold.  sale consideration 5.4lakhs.for room one.

proprtionate cost of land and cost of construction ......this may be calculated by dividnig the costs by total squarefeet construction and then multiplying the area of the room .