acknowledgment size for STOP-AND-WAIT protocol
Your equation assumes a acknowledgment packet with a size of zero and no processing time. The denominator of this equation must be extended to include also: 


the processing time \$T_{proc,R}\$ of the data packet at the receiver
the transmission time \$T_{Ack}\$ of the acknowledgement packet
the processing time \$T_{proc,S}\$ of the acknowledgement packet at the initial sender.


The time for a complete transfer (request and reply) is:

$$T_{total} = T_{Data} + T_{prop} + T_{proc,R} + T_{Ack} + T_{prop} + T_{proc,S}$$

Putting this into the efficiency equation and then transposing it to the requested acknowledgement packet size is left as an exersize to the reader.