For adding ammonia it's a simple calc
20g = 80litres or 80,000 ml
of that is .08ml
is .32ml of pure ammonia.
Of course pure ammonia is impossible to add because it is a gas at room temp and standard pressure. So bottled ammonia is dissolved in water to about 8% (for my Old Country ammonia). So you need 4ml of that to get .32 ml of ammonia. That's just about 3/4 tsp. add half and wait for it to mix and check level. You should need the other half to get to 4ppm. Then wait 24 hrs and check again. If ammonia and nitrite are both 0 yer dun.