If your ammonia reading is 8.0, not 0.8, then you need to do a 75% w/c, then do a 50% w/c not long after, to get it down at least to 1.0. Are you sure it isn't 0.8??? because I don't see how you could have any fish left at that level, if it's been staying like that for long.
If ammonia were 8.0 and you did a 50% w/c, then ammonia would immediately read 4.0, because you would be diluting it with fresh water. If your water did not test out at 4.0 right after a 50% w/c, then something could be amiss with your testing methods.
Maybe you should put your testing to the test, so to speak, by checking ammonia immediately before and after a 50% w/c. Post up your results. It might help narrow down the possible problems.