If you are considering the zero subnet unusable the 800th subnet would correspond to the binary number 800.
Use a network binary place holder worksheet and place the binary number 800 such that the least significant bit is in the place of the last subnet number (this corresponds to the position of the last 1 in the subnet mask in binary). Then interpret. The binary number 1100100000 is 800.
However if you are considering the zero subnet as the first (usable) subnet then the 800th subnet would correspond to the binary number 799 and the following would be correct. The binary number 1100011111 is 799.