if you go here, it will show you the expected height
i then looked at the month of June, basically at some period you will not be able to get in. You could also to check take portsmouth tide and change based on the secondary port calculations. I think the information at this site does this for you, but would have to check the book to be 100% sure.
Also as the previous post says some areas have a drying height that needs to be factored in
I once tried it at LW just for practice, i could not get in