You just need to go by trial and error. Raising by 1 is the safe way - but can take a while if you are far from the ideal price.

If you think your price might be way too low you could raise it by by 2n . You just raise it (by 2 or 4 or 8 etc) until stadium is not full then lower it to the mid-point between current price and previous. Just repeat this raising or lowering by half the difference, depending on whether its full or not full respectively and you'll find the price. This latter method works better and faster the bigger the gap between current ticket prices and the optimal one.

Ofc easier just to go +1 if you don't have anything to compare with.