You cannot test CYA that way--you'll get a false reading. ALL the tests are best done in daylight or sunlight and CYA has to be done in direct sun.

Meanwhile, you should go ahead and add your last two boxes of Borax. Remember: Most pool test kits cannot measure pH below 6.8. It could be 4.2 and would show as 6.8. Until you get it to 6.9, you won't know that you've reached that 6.8.