the 50/50 is merely a recommend mix but the problem is may be only the manufacturer would know the exact amount since the total volume would be the amount of water in the radiator + inside the engine. on the radiator, that's easy just remove the drain plug pero dapat sahurin mo to know the volume. the problem is yung nasa engine block, hindi lalabas yun, so unknown ang volume.

what i would suggest is just drain the radiator, then put a hose of running water into the radiator and let the engine run. this will let the water in the block to circulate. once the engine get it's normal operating temp sa gauge, shut the engine off & the water in the hose, let the radiator drain. return the radiator plug and fill the radiator with the coolant. replace distilled water dun the water reservoir and from there on, only add distilled water in case kulang sa tubig.