if you "bitxor" the pristine (transmitted) file with the recieved file, you'll get mostly zeros (which indicate no issues) and some nonzeros.
Each nonzero result is at least one bit error, (1, 2, 4, 8, 16, 32, 64, 128 all indicate one bit error, other values, you'll need to work out if they're 2,3, 4, 5, 6, 7, or 8 bit errors. (e.g. 255 would be 8)
Once you calculate how many bit errors you have, you can divide it by the number of bits sent.
