If the distance between the acceptor and donor is more than 7 nm or 70 A, that means the FRET efficiency is lower.  Energy transfer efficiency is most sensitive to distance changes when the donor-acceptor separation length approaches the Förster distance  for the two molecules. 

Please check the below link for more information

https://www.amherst.edu/media/view/106141/original/FRET.pdf

thanks kiran, 

is it possible that absorption spectra of two acceptors have almost the same overlaping extent with the fluorescence spectra of donar, but on calculation r values are different. Even calculated Energy transfer efficiency is almost same. But the binding affinity of acceptors are different for the same donor.  

The fact that a distance is less or more than 7 nm is not enough, since the Forster-type energy transfer efficiency dependence depends on the sixth power of the distance and the dynamic range is most sensitive in the range 0.5R0