Good shampoos usually have 20%-30% solid content as it is easy to be applied and rinse out from the hair.
If it doesn't have enough solid it will be too watery and wash away quickly,
similarly too many solids will be hard to work into the hair or too hard to wash out.
The percent solid contents of all the tested shampoo was found within the range of 22e25% and are expected to wash out easily (Table 2).