ecoAddRepeater -cell cell_name -net net_name -relativeDistToSink value
This command add the buffer based on the relativeDistToSink value. The value should between 0 to 0.9. A low value (0.1) places the buffer near the sink; a high value (0.9) places the buffer near the driver.
ecoAddRepeater -cell BF01D8 -net CORE/n13 -relativeDistToSink 0.5
This command add the buffer in the mid point of sink and drive.
This may helpful to you..