Instead of memorizing there are other ways to get at the notes.
- For example, when you include all the notes, lines notes and space, notes they go in alphabetical order: ABCDEFG-ABCDEFG etc. It is really handy to learn to say your 8-letter alphabet forward and backward so that you can count up and down from where you are (including known ledger lines.
- There are some spots that you can clue in on. In the bass clef, the clef sign's knobbly thing is on F, so you always know where F is. If you know middle C, then your counting the alphabet works again (C,D,E up to the next line). If you know that two ledger lines below the bass clef is C, then you can count backward and know the third ledger line must be A (C,B,A).
- Recognizing intervals is also handy. Notes on adjacent lines or spaces are a third apart so you can count up and down - C to E. If they skip a line then they are a fifth apart which is the distance of the outer notes of a triad - C to G for example.
So if you use all those tricks, then if you don't know some of your ledger lines you can still figure out the notes by going from the lines you do know.