To the OP, the text identifies TWO sources of water. Your calculations (which are based on a faulty hermenutic in the first place) don't take into account how much of the water came from the "fountains of the deep." It's an indeterminate two variable problem.
"In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. And the rain was upon the earth forty days and forty nights."
(Gen 7:11-12 KJV)
"The fountains also of the deep and the windows of heaven were stopped, and the rain from heaven was restrained;"
(Gen 8:2 KJV)
"In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. And the rain was upon the earth forty days and forty nights."
(Gen 7:11-12 KJV)
"The fountains also of the deep and the windows of heaven were stopped, and the rain from heaven was restrained;"
(Gen 8:2 KJV)
