I wouldn't worry about getting 50%, never works out for me!!
I just do whatever work then put 2 1/2L in, BEFORE any water to top up, that way you know there is enough antifreeze, I think it's about 5.5L, but some will still be there in the block/matrix etc, when tested mine was ok to -45C, so think that's enough, I aint going out if that cold!!
Top up the next day when level settled to just over minimum mark(when cold), never over the max mark when cold.
Make sure the turbo return pipe to the expansion tank is clear and unobstructed as this is also the degas pipe.
tabetha