My math (good luck following it! It confuses even me!);

Start out with 50/50 mix
__________________________________________7.25L water 7.25L coolant
Remove 9 liters of mix_______________________ -4.25L water -4.25L coolant
Add 8 liters water___________________________10.5L water 2.5L coolant 19% mix (2.5L devided by 13L total amount of fluid)
Remove 8 liters of mix (61% of total fluids)_______ -6.5L water -1.5L coolant (10.5L water *.61 = 6.5, 2.5L coolant *.61=1.5)
Add 8 liters water___________________________ 12.0L water 1.0L coolant 7% mix (1L devided by 13L total amount of fluid)
Remove 8 liters of mix (61% of total fliuds)_______ -7.4L water -.6L coolant (12L water *.61=7.4, 1L coolant *.61=.6L)
Add 4L of water and 4 of coolant________________ 8.6L water 4.4L coolant 34% mix (4.4L devided by 13L total amount of fluid)

Even if you get it completely topped off (at full 14.5L) with 50/50 mix from a bottle you are only adding another .75L of coolant. This gives you a slightly better 35% mixture.