Nut, you need to consider 0.25ppm your absolute maximum level for ammonia and nitrite, then you need to do some easy maths.
If you do a 50% water change, you are taking away 50% of the ammonia and nitrite. If you have a level of 1.0ppm, then taking away half of it will leave you with 0.5ppm. If you have a level of 0.5ppm, then taking away half of it will leave you with 0.25ppm. You need to remember, though, that the level won't stay at 0.25ppm for long, it will soon build back up. So, if you have a level of 0.5ppm then you need to do a larger water change than 50%, to build in some leeway to allow levels to rise somewhat until you are able to test and change again.
Have a read of the green link in my signature area.
If you do a 50% water change, you are taking away 50% of the ammonia and nitrite. If you have a level of 1.0ppm, then taking away half of it will leave you with 0.5ppm. If you have a level of 0.5ppm, then taking away half of it will leave you with 0.25ppm. You need to remember, though, that the level won't stay at 0.25ppm for long, it will soon build back up. So, if you have a level of 0.5ppm then you need to do a larger water change than 50%, to build in some leeway to allow levels to rise somewhat until you are able to test and change again.
Have a read of the green link in my signature area.