My planters are 6in terra cotta standard flower pot style. So, just under 5 cups volume. I water with at least 3.5 cups of water in 2 stages a few minutes apart. Water drains freely both times. If I don't get a good sized puddle the second time, I add more water after a few minutes.
Sometimes I submerge the pot up to just below the rim and let it soak for 5 to 10 minutes, just in case there are stubborn dry spots. It's a habit I picked up from the few pots that still have sphagnum peat soil.
The soil consists of 1 part coconut coir, 1 part perlite, 2 parts mushroom compost (which had a large portion of coarse sand already, maybe 30-50%), and 2 parts coarse sand... approximately. When this soil dries out, it does not compact or solidify like some others I have used. The mushroom compost was from a local mushroom farm. It's too bad I don't live in that area any longer.
There's a chance my watering style isn't providing enough water, or that the soil doesn't hold water like it should to help the plant compensate for the intensity of the sun.