if your doing a 50% water change to lower ammonia you should wait at-least 4hrs after the water change to get a accurate reading .
a 50% water change should drop you down by 1/2
before water change 2.0
After water change 1.0
most wait between changes at least 8/12 hrs as it is less likely to shock the fish.
Me if I see a lethal levels present I always do 2 50% water changes back to back yes I will put 2 hrs in-between water changes as a buffer I feel this is enough time for the fish to settle , it's a bit more aggressive but I have used this method since day 1 with good results.
everyone has their own preference on how they perform these emergency water changes . not everyone will agree on the aggressive method as we all have our own opinions . it's more about what makes you comfortable .