If you ran the YPAO, the receiver has set the correct level for you. You don't have to do anything else. What you are trying to do is adjust the levels MANUALLY, but if you ran the full YPAO, the receiver has already set the correct sub levels for you. You don't have to do anything else!
You are looking at the LFE test menu....But if you wanted to set the levels manually, you can use the LFE test menu. That 88Hz value is the frequency of the test tone which is emitted when you select the Test Tone "On" menu item. Then you can use the arrow keys to change the frequency of the test tone from 88Hz to some other frequency while observing the levels on your SPL meter. The trick to using it is to pick two frequencies, one where the subowoofer is active and one where your mains are playing. Then compare the levels while looking at your sound meter. So if your crossover is set to 80Hz, play the tone at 50Hz, and play the tone at 110Hz and compare the levels.
But again, you don't need to do this.