Approximately 2 ml/gal on the Rhizotonic for foliar feeds, but can vary up or down based on the sensitivity of the plant to translaminer movement. The hormones and some vitamins will be what is taken up and they work at very low levels. However, if needed you can go up but the high limit will be about half of the normal amount used for root feeding. This is a growers choice. I would not apply it in flower for many reasons as a foliar. You can adjust the pH but gently and make it come in around 7 or a little higher. You only have to be close. After about the second week past flower initiation I would not use it foliar anymore but you should use it from the beginning foliar. Apply about 3 times per week or every other day.
See if the new pH ( 5.8 ) turns things around... Could also be a humidity issue, or just genetics as someone else mentioned.