Do you get runoff from watering that amount?
I have zero experience with fabric pots, but I have noticed that "dry-dry" soil can absorb about 1/4 it's volume in water before being saturated. And some folks mention watering with 5% of the container volume every day, which in your case is about 4 cups. So that's probably a good starting point. It's much harder to overwater large plants in the short term, and I'd imagine fabric pots help with that, too.
You mentioned it being hydrophobic in the other thread; when that happens you will get runoff fairly quickly, and it can help to just let it sit in a tray of water for a few hours to make sure everything gets evenly rehydrated.
Edit to add: that 5% per day number really depends on the size of the plant. I grow my bigger plants in SIPs, and just based on the time it takes them to empty the reservoir, it probably varies from 2-8% depending on plant size. 5% is probably a good starting point but be prepared to adjust from there. Either way, keeping consistent moisture level seems to work well for organic soil...