Exemption u/s 54 f

Which amount should be invested for claiming exemption u/s 54 F, whether it should be actual sale consideration or sale consideration u/s 50 C of I.T Act 1961.

And which amount should be taken for computing proportionate exemption of capital gain u/s 54 F i.e. actual sale consideration or sale consideration u/s 50 C if investment is made lower than the sale consideration.