2 lavs to 20 pax is a little generous.
77W has 4 lax for 56 - 1 per 14 pax. A380 UD is 1 per 19...it's high but not tragic. Certainly the wait could be long after the main meal and after the 40 min to landing call but manageable the rest of the time.
Most Y has between 35-50 pax per lav (BA 77W is 5 lavs to ~225 pax).