I can think of several possibilities for the differences.
1) The current algorithms have superceded the previous printed tables. You should not expect to calculate the same results as appear in older printed tables.
2) Again with regard to older printed tables, consider the following comment...
I agree with others who have already said this could work for the parameters you have provided but how well it will work for your situation will depend on the accuracy you are looking for.
You can determine the pump's volume versus speed factor from the manufacturer's data and periodically...