Well you need to deduct about 20 gallons off your tanks total volume to account for rocks and equipment in the sump plus sumps are only filled about 2/3 of their volume so my 80g system is only 60g . Tank shape is also a factor. A long shallow tank is better then a square tank so tangs can swim more horizontal. Some people are successful with undersized tanks and often show it on here but I bet a far greater number are unsuccessful but they are not going to post their failures for everyone to see. As hobbyists we have a duty to provide the best care we can for our aquatic animals.That’s so interesting as that’s a tang that’s listed for 75 gallons and under.