okay and heres my opinion with a bit of justification so you can choose better which method you think works.
ammonia and nitrite inhibit the fish's ability to breathe, using up or clinging to oxygen molecules in the water. thus why having those two in your tank water can kill fish. using prime only adds other ions or cations for ammonia to cling onto for a short period of time. but for it to be effective you must add more than just the normal dose. it is safe for you to add up to 5x the regular dose with no ill effects to fish.
if you want to get rid of ammonia or nitrite in your water and want to know how much water to change to get one of the two to a specific level try this conversion.
if you want to reduce either by 50% (from .25ppm to .125ppm) you must do a 50% WC
to get it down to 0ppm you must change 100% of the water
if you do only 20% WC it will go from .25ppm to only .20ppm
not much of a difference.
now i wouldnt do a water change in a tank if ammonia is <.25 mg/l or ppm same goes for nitrite. b/c you must do LARGE WC to get it to a very low level as you saw in the math above.